Mossing for Gold

When I first arrive at a new place to pan for gold, one of the first things that I like to do is find a thick patch of moss near the water line and take a sample of the material underneath.

Cleaning the moss can be a little tedious as there tends to be a lot of silt and clay and the gold is often very fine flood gold.

The reason that I like to moss for gold is that it has proven to be a good indicator of what is happening in the area.

Because there tends to be more fines than larger gold, I like to use the mossing as a way to determine whether or not I will start a more expansive sampling process or whether I will look for better places.

Another advantage to mossing for gold is that it is a quick way to see some color in the gold pan. This always starts my prospecting day off with some excitement and gets the gold fever stirring.

When you go gold panning, do you moss? How do you start off your prospecting trip?

Sampling for Gold

When you are into the gold deposits, gold prospecting can be extremely fun. Many times, however, prospecting can be frustrating when little gold appears to be present. When this is the case, it is often worth it to spend your time sampling for a better location before getting out all of your gold panning equipment. Let me give you a small example to illustrate the need for sampling.

On two separate trips to a particular location that was noted for having gold, I spent a lot of time working up and down the small runoff creek with little success. Every now and then I would come across a speck of fine gold and maybe a flake if I was lucky. I sampled what seemed to be all of the probable and obvious locations for finding gold deposits, but continued to find meager amounts of gold. I panned some of the gravel from along the banks and still not much gold.

After this second trip, I decided that I wouldn’t go back to that particular location. A few months later, I was talking about gold panning in this creek and he insisted that there was some good color if you knew where to look. I decided to give the place a last chance and went back, this time accompanied by my friend. We again ran a few pans from the creek with little to show, when he suggested a place that had provided some good color in the past. It was about a 75 yards away from the creek in a completely dry location.

Rather than pan it out, he set up a sluice and we began filling buckets with gravel. At this point, I was a little more optimistic because I could see clear evidence of drywasher tailings nearby. Sure enough, this little area produced a decent amount of gold flakes and even a small picker. The lesson I learned that day was that even if the pickings seem slim in all of the right locations, it can be worth it to run some samples away from the river, what may seem a poor place to pan can actually hold a pleasant surprise. Since then, I am much more open to expanding my search areas.

How to Cleanup a Sluice Box

When using a sluice box, you will be able to process a lot more material in a shorter amount of time than you could accomplish with a gold pan. This means that your sluice box can accumulate a large amount of black sand and hopefully gold in a short amount of time. Failure to clean your sluice box regularly can result in lost gold as the riffles can become so bogged down with heavy concentrates that the newly added gold bearing gravel merely washes out of the sluice about as fast as it is added.

So how do you know when it is time to cleanup your sluice box? Generally, I like to cleanup when black sand begins cover much of the top three riffles. If you can no longer see some of the riffles and instead you are only able to see black sand, then this is bad.

It is hard to say how often you should cleanup your sluice box because every location is different. Some gold bearing rivers have little black sand and other have large amounts of heavy concentrates. In the first type of location you may be able to sluice for gold all day without cleaning up, but in the second type of location yo may have to cleanup every hour or two.

When cleaning up your sluice box, the first thing that you need to do is set up something to catch all of the contents of sluice. This can be a gold pan, a bucket or perhaps one of those black plastic masonry tubs. I prefer something deep like a bucket which makes it more difficult for the concentrates to splash out of.

Next you will need to pick up the sluice box slowly and keep it level. You want the water to drain off the lower end of the sluice without washing out much of the material in the sluice.

Then you will places the lower end of the sluice into the bucket, gold pan or other container and tip the sluice up at an angle. With another gold pan or small bucket, pour water into the top of the sluice to wash out the bulk of the gravel and concentrates. Now you can unlatch the riffles and lift them up. If the riffles come out completely, be careful not to knock the material off as gold may be stuck to it. Rinse the the riffles off into the bucket and then set it aside. If it is attached to the sluice, rinse them off and then lift them out of the way.

Now carefully remove the carpet or miners moss from the sluice and keep the whole thing in the bucket and then pour more water down the trough of the sluice to rinse out the remaining concentrates. You may need to scrub off some clay buildup which may have caught some of the gold.

After removing the clean sluice from the bucket, it is time to clean the carpet material. Carefully grab each end of the carpet and while keeping your hands close together, lift up the ends of the carpet. As most of the fines will be in the carpet, be especially careful to keep the carpet in the bucket area so that any material that drops off will fall into the bucket.

At this point, the carpet should be in the shape of a “U” with the middle sagging into the bucket. Lower the carpet until the middle is submerged in the water. Carefully lower one end of the carpet while lifting the other side so that one half of the carpet passes under the water. Repeat in the opposite direction. Turn the carpet over and do the same thing to the other side of the carpet. This will allow the heavy material to fall out of the carpet and end up in the bucket. Repeat the process until the carpet is clean.

Now you have a choice, you can either pan down the concentrates to collect your gold or you can dump keep it in the bucket and cleanup later. I prefer to maximize my time when gold prospecting so I prefer to save up all the concentrates and clean them up at a later time.

Finally, you can put your sluice back together and get back to sluicing for gold.

Gold Pans – A Buyers Guide

Gold pans come in a variety of sizes and shapes with several different features. However, the purpose and function of these gold pans is the same. A gold pan’s primary purpose is to classify gold bearing gravel so that the heaviest materials (such as gold) end up in the bottom of the gold pan, while the ligher materials are washed out of the gold pan.

Though all gold pans can work, I have created this guide to help you understand why I prefer to use the Garrett Gravity Trap style gold pans. Along the way, you may discover a type of gold pan that will better fit your needs. This is perfectly ok. What is important is that you find out what works for you.

When I first got into gold panning, I was given a generic plastic gold pan and it worked. It had tiny bumps for riffles and it kept gold in the pan. In reading some articles from old-timers, I discovered that some of them preferred the traditional steel gold pans and each had their way of bluing the gold pan with heat to remove the oil film on the steel and to make it easier to see gold. I figured that if the oldtimers preferred steel, then I should give it a try.

gravity trap gold pan

Right away I noticed two main differences between the steel gold pan and the plastic. First, the steel pan was heavier and as a result, it was more tiring. Second, in cold water the steel gold pan felt like I was holding onto an ice cube. Yes the steel pan found gold and is stronger than a plastic gold pan (I have never broken a plastic one), but I think that die hard steel gold panners base their decision to use steel on ego or to be an old-timer more than on logic. Just my opinion. Regardless, I decided that plastic was the way to go.

With plastic gold pans, there are a few different shapes–typically round and a variety of rectangular shaped pans. Round

gold pans make for an easy swirling motion or side to side motion rather than just the side to side motion of square pans. Two of the most common square pans are the Le Trap and the Grizzly gold pans. The Le Trap looks similar to a traditional pan, only square. It works well enough, but I like the flexibility of different panning motions provided by the round pans and I also like using the back of the gold pan for cleanups. Seems more natural to use a round pan.

grizzly gold pan

The other type of square gold pan is the Grizzly gold pan. Looking from the side, the Grizzly gold pan is shaped like a “V”. It uses a strict side to side motion and the gold concentrates are emptied through a plug in the bottom of the pan. While this pan does process material faster than a regular gold pan (the advantage) it also has several disadvantages. First, the Grizzly gold pan requires deeper water for use. You must be able to submerge the pan completely. This is not necessary with a traditional pan. Second, the “V” shape makes it awkward to set down. Third, the plug is easy to lose and only allows quarter inch or smaller material to pass through. Ffourth, the Grizzly gold pan is made of a plastic that feels weaker than other gold pans.

Ultimately, I prefer to use a round style gold pan. With round gold pans, there are also some different styles and sizes to choose from. When choosing a size, I have found that the 10 to 14 inch diameter gold pans are just the right size. The 10 inch size is great for cleanup and the 14 inch is perfect for production. Smaller than these sizes and it is difficult to process much material. Larger than this size of pan and the weight of the gold pan with gravel becomes tedious and can actually slow you down and wear you out faster. Some 14 inch gold pans have a wider bottom than others. I prefer a medium width bottom as too wide and the weight becomes too much.

trinity gold pan

Some round gold pans are completely smooth, some have small riffles, medium riffles, large riffles, and even recessed areas in the pan that can all affect gold panning outcomes. The Trinity gold pan is an example of a gold pan that has a recessed area. I do not like this feature because I believe that it interferes with the classifying action and may result in loss of gold as the trap is in the back of the pan which is usually the higher side of the pan when being used. If gold pops out of the small trap, it can possibly ride down the top of the light material and out of the pan.

Small riffles can be effective for catching really fine gold, but they can also be more difficult to cleanup as the small gold and clay can really get stuck in the riffles. I do not like to waste time cleaning out the riffles regularly when panning and so I no longer use a gold pan with tiny riffles. Some gold pans like the Garrett Super Sluice pan have really big riffles that are supposed to allow you to process more material. Personally, I prefer the medium sized riffles as there are a enough riffles to catch the gold and even if a little clay builds up, they will still catch the gold, even the fine gold.

Now I have mentioned that I like the Garrett Gravity Trap gold pans over other available options. The reason for this is that their style of riffles appeals to me and the gold pan has been around for awhile. Some other manufacturers have made similar designs that will work as well. I simply purchased a Garrett pan before trying one of the very similar designs and saw no need to purchase a pan that was so close in design to one that I own. So regardless of the brand, it is the Garrett Gravity Trap style of gold pan that I like, recommend and use most of the time. Sometimes I will still use the Grizzly Gold Pan when I cannot bring in a sluice.

Gold Prospecting Classifiers

Now would also be a good time to mention gold prospecting classifiers. My favorite classifier is by Garrett. The Garrett Gold Classifier is constructed of solid plastic and is designed to work well with most conventional 14 inch gold pans. The 1/2 inch holes are large enough to prevent most gold nuggets from being tossed while getting rid of all of the large rocks. This speeds up the gold panning process.

Another advantage of the Garrett Classifier is that it stores easy will my other gold pans. Some of my other classifiers are wider and more awkwardly shaped.

At half the price of many of its competitors, the Garrett Gold Classifier is definitely one of my favorite buys. They are tough and really get the job done. The only downside to Garrett Classifiers is that they only come with the 1/2 inch holes. I also like to have a 1/4 inch and a 1/8 inch gold classifier on hand. But, if I could only choose one gold classifier, then it would be the Garrett Gold Classifier.

How to Find Gold in A River: A Gold Prospecting Guide

When gold prospecting a new stretch of river, it can at times be difficult to pick a place to start gold panning. There are a number of places that are good starting points for any search. I will cover how to read a river for gold and how to sample a stretch of river.

Placer gold tends to deposit in areas of low water pressure. By this, I mean that anywhere the water slows, gold will try to find a resting place, while lighter gravels continue to wash down a river. When searching for these low pressure areas it is important to visualize the river during the runoff season as this is the time when the gold moves the most. If you are not able to visit the river during high water season, look for the high water mark. Look for the highest points on the bank where drift wood has been deposited. This is probably close to the high water mark.

Gold often travels down the gut of the river, taking the shortest path possible. This can be visualized by looking from the inside edge of a bend in the river up or down to the edge of the next bend. Look for anything that might obstruct the flow of water along this path. There might be a large boulder or other object that can break the flow of water. The down stream side of the object can catch gold.

Look for sections of bedrock that cross the river. If there are cracks or crevices crossing the flow of water, they can act as natural riffles in a sluice box. Surprisingly large pieces of gold can work their way into what appear to be small cracks. When a river widens, the water pressure decreases. This can allow gold to deposit.

The inside bend of a river causes water to slow enough that gold can be deposited on the bend along with other gravels that form a bar. Depending on the bar, placer gold can be concentrated on the up-river and down-river sections of the bar.

It can also be deposited in pockets throughout the bar. Flood gold, the gold that is easily moved during floods, can be scattered in the top layers of gravel thoughout the river with pockets forming in spotty locations. These are not the only places that placer gold can deposit, but they are some good places to start.

Now I highly recommend that you try sampling rather than just picking a spot and going for it. The goal of sampling is to locate a more richer section of gravel before you start gold panning. It is well worth the time and effort to sample so that you can find the pockets or paystreaks of gold. A paystreak is often a somewhat narrow stretch of river that has a higher concentration of gold than the rest of the river bed.

When sampling, don’t wast time trying to extract every flake or speck of gold from your gold pan. Look for a promising spot and pan it down until you can get a good idea of the number of pieces of gold that came from the hole. Make a note of the spot and move onto another promising spot. Continue to make note of the concentration of gold in each spot. Then, after sampling several spots, go back and work the spot that was producing the most gold.

If while sampling you discover a really good deposit. Take some samples upstream, downstream, and on the sides to find the boundaries of the deposit. This will let you know the general area of the gold deposit and you might just discover an even richer part of the deposit. I know that I would rather work the richest part of the deposit, if I was limited on time.

Once you have located a good spot, now is the time to slow down and really work the area. Pan the material down until you only have about a half a cup of material in your gold pan. At this point, you can see some of the gold and know whether or not you are still in a productive spot. Rather than spend a lot of time separating the gold from the other gravel, place it in a small bucket or container to cleanup at home. This way you spend more time getting the gold.

Submarine Volcanoes, Hydrothermal Vents and Biodiversity Discoveries in the Sulawesi Sea off the coast of Indonesia

Submarine Volcanoes and a Hydrothermal Field

The white outline on the map shows the operating area where both Indonesian Research Vessel Baruna Jaya IV and NOAA Ship Okeanos Explorer conducted joint operations in 2010. The expedition focused on the diversity and distribution of deep sea habitats and marine life in unknown ocean areas in SATAL - a contraction of Sangihe and Talaud - two island chains stretching northeast of North Sulawesi in Indonesia. Image courtesy of NOAA Office of Ocean Exploration and Research.

New submarine volcanoes, a large hydrothermal field with a thriving exotic animal ecosystem and areas rich with deep-sea ocean animals are among the discoveries reported today by U.S. and Indonesian scientists who explored the largely unknown deep Sulawesi Sea last summer off the coast of Indonesia.

At an American Geophysical Union press conference in San Francisco, scientists explained that while the exploration area is recognized as one of the Earth’s major shallow water centers of marine diversity, little was known about the marine life inhabiting its deep areas until this mission.

Mapping Kawio Barat: An Undersea Volcano

“This expedition was exciting and productive in many respects,” said Sugiarta Wirasantosa, Ph.D., of the Indonesia Agency for Marine and Fisheries Research and Indonesia’s chief scientist for the expedition. “The joint science team mapped Kawio Barat, an active undersea volcano that rises nearly 12,000 feet from the seafloor, and the mission revealed that high marine diversity extends deep in the area, but that there is a different mix of diversity between the shallow and deep ocean.”

Biodiversity in the "Coral Triangle"

“Within the ‘Coral Triangle,’ a 2.3 million-square-mile area (6 million square km) in which the Sulawesi is included, more than 65 percent of the world’s reef-forming coral species are known to exist in shallow waters,” said Santiago Herrera, a graduate student at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution who participated in the expedition. “We observed and imaged perhaps 40 potential new coral species and 50 potential new species of other animals, including those inhabiting an actively venting volcano. Documenting the abundance, biodiversity and distribution of deep-ocean animals will allow us to better understand the functioning of the ecosystems in the area and infer how resilient they are to human activities.”

The ocean exploration partnership matured in the wake of President Obama’s speech in June 2009 in Cairo when he spoke of building partnerships to support science and technological development in Muslim-majority countries. This expedition was the first in a multiyear plan for Indonesia and the United States to explore marine environments together as part of a larger partnership that foresees NOAA, Indonesia’s Ministry of Marine Affairs and Fisheries, and Indonesia’s Agency for the Assessment and Application of Technology partnering on issues of mutual interest including ocean exploration, fisheries and food security, climate change and tsunami research, among other areas.

Hydrothermal vents were found during the second ROV dive on Kawio Barat volcano. The yellow deposits are molten sulfur and multiple species of hot-vent shrimp are also visible. Image courtesy of NOAA Okeanos Explorer Program, INDEX-SATAL 2010.

close-up imagery showing a type of goose-neck barnacle, shrimp and a scaleworm living more than 1,850 meters deep on Kawio Barat underwater volcano. Image courtesy of NOAA Okeanos Explorer Program.
Maiden Voyage of the Okeanos Explorer

The 2010 expedition was the maiden voyage of NOAA Ship Okeanos Explorer, which worked with the Indonesian Research Vessel Baruna Jaya IV. U.S. and Indonesian scientists worked side-by-side on both ships as well as in shore-based Exploration Command Centers in Jakarta and Seattle where they received information in real-time via satellite and high-speed Internet2 pathways, including high-definition video of the seafloor from the Okeanos Explorer’s remotely operated vehicles. Other scientists were on call ashore to assess the data, information and images as needed. At other Exploration Command Centers, including one in Silver Spring, Md., and one at The Inner Space Center at the University of Rhode Island in Kingston, live video came in from sea via telepresence technology and engaged a variety of audiences ashore.

“Our partnership to explore the ocean and to share knowledge and technology advances science while building and strengthening the friendship between our nations,” said Jane Lubchenco, Ph.D., the U.S. under secretary for oceans and atmosphere and NOAA administrator. “We look forward to further cooperation next summer when the NOAA Ship Okeanos Explorer is scheduled to return to Indonesia to continue this mission.”

Indonesia: A Nation of 17,000 Islands

“It’s especially important for Indonesians to better understand our ocean,” said Sugiarta. “Indonesia is a nation of 17,000 islands with a population that depends largely on the ocean for safety and on ocean resources for food, trade and economic well-being. Measurements of the flow of deep water masses through the deep Sulawesi Sea will help us better understand the ‘Indonesian Throughflow,’ which is important to all because it plays a major role in the global distribution of heat transported by ocean currents.”

Recent Volcanic Activity at 6,200 Feet
“We had a fantastic view of the summit area of Kawio Barat and the features we saw strongly suggest very recent volcanic activity at 6,200 feet (1,900 meters),” said David Butterfield, PhD., a scientist with NOAA’s Pacific Marine Environmental Laboratory in Seattle. “Seeing an eruption at Kawaio Barat is a priority for future observations. Although 70 percent of Earth’s volcanic activity takes place under the ocean surface, researchers have only observed active eruptions by two undersea volcanoes.”

Telepresence Technology

The application of telepresence technology for ocean science and exploration and for education and outreach was first envisioned by Robert Ballard, Ph.D., who partnered with NOAA to develop and refine the technology to bring the excitement of discovery in real time to audiences ashore. Expedition scientists on this latest mission believe that high-definition video transmitted from the deep sea to scientists ashore in real time provided a significant step forward in identifying marine animals, geologic features and other aspects of the deep regions of the Sulawesi Sea.

“In an incredible extension of telepresence technology, live images from the seafloor also went for the first time to scientists ashore beyond Exploration Command Centers,” said NOAA scientist Steve Hammond, Ph.D., the expedition’s U.S. chief scientist. “One scientist at the University of Victoria shared the live seafloor video with her ocean science students and took still frames from the video to email to other ocean experts who could help with identifications. We had scientists of many disciplines in numerous locations all sharing comments in an online chat room as they viewed live video,” he said. “All those comments are time-coded to the video for further reference and research.”

Expedition Partnership and Support

Sea World Indonesia in Jakarta and the Exploratorium in San Francisco were education partners in the expedition. A chronicle of the expedition, including logs and images from sea, is available online.

Celebrating 10 years of ocean exploration, NOAA's Office of Ocean Exploration and Research uses state-of-the-art technologies to explore the Earth's largely unknown ocean in all its dimensions for the purpose of discovery and the advancement of knowledge.

NOAA Ship Okeanos Explorer is operated, managed and maintained by NOAA’s Office of Marine and Aviation Operations, which includes commissioned officers of the NOAA Corps and civilian wage mariners. NOAA’s Office of Ocean Exploration and Research is responsible for operating the cutting-edge ocean exploration systems on the vessel. It is the only federal ship dedicated to systematic exploration of the planet’s largely unknown ocean.

NOAA’s mission is to understand and predict changes in the Earth's environment, from the depths of the ocean to the surface of the sun, and to conserve and manage our coastal and marine resources. Find us online and on Facebook.

The Geology of Sulawesi Island

SULAWESI ISLAND

Geologically, Sulawesi Island and its surrounding area is a complex region. The complexity was caused by convergence between three lithospheric plates: the northward-moving Australian plate, the westward-moving Pacific plate, and the south-southeast-moving Eurasia plate. Regional structures, which affects the island of Sulawesi and the surrounding area, are shown in Figure 8.1. The Makassar Strait, which separates the Sunda Platform (part of the Eurasia Plate) from the South Arm and Central Sulawesi, formed by sea-floor spreading originating in the Miocene (Hamilton, 1979, 1989; Katili, 1978, 1989). North of the island is the North Sulawesi Trench formed by the subduction of oceanic crust from the Sulawesi Sea. To the southeast convergence has occurred between the Southeast Arm and the northern part of the Banda Sea along the Tolo Thrust (Silver et al., 1983a, b). Both major structures (the North Sulawesi Trench and Tolo Thrust) are linked by the Palu-Koro-Matano Fault system. Based on lithologic association and tectonic development, Sulawesi and its surrounding islands are divided into 3 geological provinces (Fig. 8.2): (1) the Western Sulawesi Volcanic Arc; (2) the Eastern Sulawesi Ophiolite Belt and its associated pelagic sedimentary covers; and (3) continental fragments derived from the Australian continent (Hamilton, 1978, 1979; Sukamto and Simandjuntak, 1983; Metcalfe, 1988, 1990; Audley-Charles and Harris, 1990; Audley-Charles, 1991; Davidson, 1991). The contacts between those provinces are faults.

WESTERN SULAWESI VOLCANIC ARC

The Western Sulawesi Volcanic Arc extends from South Arm through the North Arm (Fig. 8.2). In general, the arc consists of Paleogene-Quaternary plutonic-volcanic rocks with Mesozoic - Tertiary sedimentary rocks and metamorphic rocks. In this chapter the stratigraphy is divided into South Sulawesi and North Sulawesi system.

SOUTH SULAWESI

The Geology of eastern and western South Sulawesi is distinctly different, and these two areas are separated by the NNW-SSE trending Walanae Depression. South Sulawesi is structurally separated from the rest of the Western arc of Sulawesi by a NW-SE trending depression which passes through Lake Tempe (Fig 8.3, van Leeuwen, 1981). Figure 8.4 is a compilation of the formation names and ages of lithologies in South Sulawesi, used by various workers. For reference a geological map and the stratigraphy of South Sulawesi are presented in Fig 2.4 & 2.6. The following sections describe the geology of South Sulawesi through time.

Mesozoic basement complex

The basement complex is exposed in two areas: in the western half of South Sulawesi near Bantimala and Barru, and consists of metamorphic, ultramafic and sedimentary rocks (Fig. 8.4). Metamorphic lithologies include amphibolite, eclogite, mica-schists, quartzites, chlorite-feldspar and graphite phyllites (t’Hoen & Zeigler, 1917; Sukamto, 1975; 1982; Berry & Grady, 1987). K/Ar dating on muscovite-garnet and quartz- muscovite schists, both from the Bantimala basement complex yielded 111 Ma (Obradovich, in Hamilton, 1979) and 115 + 7 Ma (Parkinson, in Hasan, 1991) respectively. Wakita et al.,(1994) have dated five schist samples from the Bantimala complex and one from Barru complex using K/Ar analyses and yielded an age of 132- l 14 Ma and 106 Ma respectively. This data suggests a late early Cretaceous age for the emplacement of the basement in south Sulawesi. The sequence unconformably overlying and tectonically intercalated with the metamorphic lithological units consists of red and grey siliceous shales, feldspathic sandstones and siltstones, radiolarian cherts, serpentinized peridotite, basalt and diorites (Sukamto, 1975; 1982; Hamilton, 1979; van Leeuwen, 1981; Wakita et al., 1994). Radiolaria extracted from the cherts have been dated as late Albian (latest early Cretaceous: Pessagano, in Sukamto, 1975) or late Albian to early Cenomanian (Wakita et al., 1994). The presence of similar metamorphic rocks in Java, the Meratus mountains in SE Kalimantan and Central Sulawesi suggest that the basement complex in south Sulawesi may be a dismembered fragment of a larger early Cretaceous accretionary complex (Parkinson, 1991).

Late Cretaceous sedimentation

The late Cretaceous sediments include the Balangbaru (Sukamto, 1975;1982; Hasan, 1991) and Marada Formations (van Leeuwen, 1981) in the western and eastern parts of west South Sulawesi respectively (Fig. 8.4). The Balangbaru Formation unconformably overlies the basement complex and is composed of interbedded sandstones and silty-shales, with less important conglomerates, pebbly sandstones and conglomeratic breccias (Sukamto, 1975; 1982; Hasan, 1991). The Marada Formation consists of an arenaceous succession of alternating impure sandstones, siltstones and shales (van Leeuwen, 1981). The sandstone are mostly feldspathic greywacke which are locally calcareous composed of subangular to angular grains of quartz, plagioclase, and orthoclase with subordinate biotite, muscovite and angular lithic fragments embedded in a matrix of clay minerals, chlorite and sericite (van Leeuwen, 1981). Graded bedding is occasionally present in sandstone and the sandstone and siltstone. Coarser units of the Balangbaru Formation contain sedimentary structures typical of gravity flow deposits, including the chaotic fabric of debris flows, graded bedding and sole marks indicative of turbidites (Hasan, 1991). The lithologies and fauna of the Balangbaru and contemporaneous Marada Formations to the east (van Leeuwen, 1981; Sukamto, 1982) are typical of an open marine, deep neritic to bathyal environment (van Leeuwen, 1981; Sukamto, 1982; Hasan, 1991). The Marada Formation is interpreted to be the distal equivalent of the Balangbaru Formation, based on lithological and grain size considerations (van Leeuwen, 1981). The tectonic setting of the Balangbaru Formation is interpreted to be a small fore-arc basin on the trench slope (Hasan, 1991).

Paleocene volcanism

Volcanics of Paleocene age occur in restricted areas of the eastern part of South Sulawesi and unconformably overlie the Balangbaru Formations (Sukamto, 1975). In the Bantimala region these volcanics have been called Bua Volcanics (Sukamto, 1982); Langi Volcanics in Biru area (van Leeuwen, 1981; Yuwono et al., 1988). This formation consists of lavas and pyroclastic deposits of andesitic to trachy-andesitic composition with rare intercalations of limestone and shale towards the top of the sequence (van Leeuwen, 1981; Sukamto, 1982). Fission track dating of a tuff from the lower part of the sequence yielded a Paleocene age of + 63 Ma (van Leeuwen, 1981). The calc-alkaline nature, and enrichment of certain light rare earth elements, suggests that the volcanics were subduction related (van Leeuwen, 1981; Yuwono, 1985), probably from a west dipping subduction zone (van Leeuwen, 1981).

Eocene to Miocene volcanism and sedimentation

The Malawa Formation is composed of arkosic sandstones, siltstones, claystones, marls and conglomerates, intercalated with layers or lenses of coal and limestone. This formation occurs in the western part of South Sulawesi and unconformably overlies the Balangbaru Formation and locally the Langi Volcanics (Fig. 2.5, Sukamto, 1982). A Palaeogene age for this formation is inferred from palynomorphs (Khan & Tschudy, in Sukamto, 1982) whilst ostracods suggest an Eocene age (Hazel, in Sukamto, 1982). The Malawa Formation is inferred to have been deposited in a terrestrial/marginal marine environment passing transgressively upwards into a shallow marine environment (Wilson, 1995). The Tonasa Limestone Formation conformably overlies the Malawa Formation or the Langi Volcanics. This Formation consists of four members ’A’, ’B’, ’C’ and ’D’ from bottom to top. The ’A’ member comprises well bedded calcarenite, the ’B’ member is composed of thickly-bedded to massive limestone, the ’C’ member consists of a thick sequence of detrital limestone with abundant foraminifera and the ’D’ member is characterised by the abundant presence of volcanic material and limestone olistoliths of various ages (van Leeuwen, 1981; Sukamto, 1982). The age of the Tonasa Formation is Eocene to middle Miocene (van Leeuwen, 1981; Sukamto, 1982; Wilson, 1995). A ramp type margin is inferred for the southern margin of the Tonasa Formation, and the Tonasa Carbonate Platform is composed mainly of shallow water facies, whilst redeposited facies predominated the northern margin (Wilson, 1995). The Malawa and Tonasa Formations have a widespread distribution over the western part of South Sulawesi (Wilson, 1995). These formations do not outcrop east of the Walanae Depression (Fig. 2.4.) apart from a small outcrop of the Tonasa Limestone Formation at Maborongnge (Sukamto, 1982; Wilson, 1995). The Salo Kalupang Formation is present in the eastern part of South Sulawesi (Fig. 2.4). This formation consists of sandstones, shales and claystones interbedded with volcanic conglomerates, breccias, tuffs, lavas, limestones and marls (Sukamto, 1982). Based on foraminifera dating techniques, the age of the Salo Kalupang Formation is believed to range from the early Eocene to the Late Oligocene (Kadar, in Sukamto, 1982 and Sukamto & Supriatna, 1982). This formation is contemporaneous with the Malawa Formation and the lower part of the Tonasa Formation (Sukamto, 1982). The Kalamiseng Formation outcrops to the east of the Walanae Depression (Fig. 2.4) and comprises of volcanic breccias and lavas, in the form of pillow lavas or massive flows. These are interbedded with tuffs, sandstones and marls (Sukamto, l982; Sukamto & Supriatna, 1982; Yuwono et al., 1987). The lavas are characterised by spillitic basalts and diabases which have been metamorphosed to a greenschist facies (Yuwono et al., 1988). The Bone mountains have been interpreted as part of an ophiolitic sequence based on high gravity anomalies and the marine MORB nature (Yuwono et al., 1988). K/Ar dating on pillow lavas of the Kalamiseng Formation gave late Eariy Miocene ages (17.5+ -0.88 and 18.7+ -0.94, Yuwono et al., 1988) and this may represent an emplacement age of the suggested ophiolitic suite (Yuwono et al., 1988). Intrusive bodies are exposed in the eastern part of the Biru area and Tonasa-I (Sukamto, l982) where dating by fission track yielded an age of Early Miocene (van Leeuwen, 198l). Yuwono et al., (1987) relate these intrusive bodies to calc-alkaline volcanics in the lower member of Camba Formation and suggests that both were derived from early Miocene subduction. However, this is inconsistent with a mid- Miocene (Sukamto & Supriatna, l982) or middle to late Miocene age (Sukamto, 1982) suggested by foraminifera in marine sediments interbedded with the volcaniclastics. The lower member of the Camba Formation consists of tuffaceous sandstone, interbedded with tuff, sandstone, claystones, volcanic conglomerates and breccia, marls, limestones and coals (Sukamto, 1982; Sukamto & Supriatna, 1982). The Bone Formation has been reported by Grainge & Davies (1985) from the Kampung Baru-I well in the Sengkang area (Fig. 2.5) where it comprises bioclastic wackestone and fine grained planktonic foraminifera packstones interbedded with calcareous mudstone. The limestones have been dated as early Miocene (N6-N8) in age (Grainge & Davies, 1985).

Miocene to Recent volcanism and sedimentation

The upper member of the Camba Formation described here as the Camba Volcanics, is located in the Western Divide Range forming the ’backbone’ (Fig. 2.4). This member consists of volcanic breccias and conglomerates, lavas and tuffs interbedded with marine sediments (Sukamto, 1982; Sukamto & Supriatna, 1982). Foraminiferal dating suggests a middle to late Miocene age (Sukamto, 1982) for the Camba Volcanics. The Lemo Volcanics unconformably overlie the upper Miocene Walanae Volcanics in the Biru area (van Leeuwen, 1981), K/Ar dating for Lemo Volcanics yielded an age of Pliocene (Yuwono, et al., 1988). Although Sukamto (1982) included the Lemo Volcanics as part of the Camba Volcanics, this is unlikely since the age range of Camba Volcanics is only up to the late Miocene. The lower part of Camba Volcanics (Fig. 2.5) is thought to be equivalent to the mid- Miocene Sopo Volcanics in the Biru area (van Leeuwen, 1981). The upper part of the Camba Volcanics is thought to be analogous to the Pammesurang Volcanics from the Biru area, described by van Leeuwen (1981). Yuwono et al., (1988) subdivided the Camba Volcanics into two members: Camba IIa of alkali potassic nature and Camba IIb of alkali ultrapotassic nature. Based on K/Ar dating the age of the Camba II Volcanics is determined as late Miocene (9.91 + 0.5 Ma – 6.27 + -0.31 Ma, Yuwono et al., 1988). The volcanic units of Miocene to Pleistocene age in South Sulawesi have been discussed by Yuwono et al., (1987). These includes the Baturape volcanics, a series of alkali potassic extrusive and intrusive lithologies, where K/Ar analyses yields 12.8 + 0.64 Ma (mid Miocene, Yuwono et al., l988); the Cindako volcanics have the same characteristics as the Baturape Volcanics, but K/Ar dating yielded an age of 8.2+ 0.4l Ma for the Cindako Volcanics (late Miocene, Yuwono et al., 1987). These two volcanic units are grouped together by Sukamto (l982) who suggested an upper Pliocene age on the basis that they both unconformably overlie the Camba Formation. The Soppeng Volcanics are inferred to have a late Miocene age (Yuwono et al., 1987), however, Sukamto (1982) interpreted these volcanics as early Miocene in age since they are conformably overlain by rocks of the Camba Formation. The Parepare Volcanics are remnants of a strato-volcano composed of alternating lava flows and pyroclastic breccias dated by K/Ar analyses as late Miocene (Yuwono et al., 1987). The lavas are intermediate to acidic in composition (Yuwono et al., 1987). The Plio/Pliestocene volcanics of the strato volcano of Lompobatang occupies the southern-most portion of south Sulawesi rising to a height of 2,871 m. These volcanics consist of silica undersaturated in alkali potassic and more acidic silica saturated shoshonitic lava flows and pyroclastic breccias (Yuwono et al,, 1987). The mid-Miocene to Pleistocene volcanic rocks in South Sulawesi, including the upper member of the Camba Formation, have a predominantly alkaline nature, interpreted by Yuwono et al., (1987), as a result of partial melting of the upper mantle (phlogoplite- bearing peridotite) which was previously enriched in incompatible elements by metasomatism’ (Yuwono et al., 1987). This may possibly have been linked to previous subduction in early Miocene times in a ’distensional intraplate context’ (Yuwono et al.,1987). Van Bemmelen (1949) suggested that the alkali nature of these volcanics is caused by ’excessive assimilation of the older limestones into melt’ and incorporation of continental material into a subduction-related volcanic arc (Katili, 1978). Neogene magmatism in western central Sulawesi has been related to lithospheric thickening and melting (Coffield et al., 1993; Bergman et al., 1996). The bimodal nature of Neogene igneous lithologies in this area is thought to be from the melting of ancient mantle peridotite and crust yielding alkaline basaltic (shoshonitic) and granitic composition melts respectively (Coffield et al., 1993; Bergman et al., 1996) The late Miocene sedimentation is marked by the development of the Tacipi Formation (see section 2.3.2). The Middle Miocene-Pliocene (Grainge & Davies, 1983) Tacipi Formation forms the subject of the present study and is therefore not discussed further in this section. The Walanae Formation (see section 2.3.2) is locally unconformable on the Tacipi Formation and in places, the two units interdigitate. The Walanae Formation is dated as mid-Miocene to Pliocene (N9-N20 Sukamto, 1982) based on foraminifera, or alliteratively as predominantly Pliocene (up to N2l), with the basal units probably Late Miocene in age (Grainge A Davies, 1985). In the East Sengkang Basin the Walanae formation can be divided into two interval: a lower interval made up of calcareous mudstone and an upper interval which is more arenaceous. The lower intervals outcrops intensively in the southern part of the basin which in places interfinger with reef talus of the Tacipi Formation. Limestones on the southern tip of South Sulawesi (Figs. 2.2; 2.5) and on the island of Selayar are named the Selayar Limestone which is a member of the Walanae Formation (Sukamto & Supriatna, 1982). The Selayar Member is composed of coral limestone and calcarenite with intercalations of marl and calcareous sandstones. This carbonate unit ranges from upper Miocene to Pliocene in age (N16-N19, Sukamto & Supriatna, 1982). Sukamto & Supriatna (1982) reported that an interfingering relationship between the Walanae Formation & Selayar limestone occurs in the Selayar Island. Terrace, alluvial, lacustrine and coastal deposits occur locally in South Sulawesi. Recent uplift in South Sulawesi is characterized by raised coral reef deposits (van Leeuwen 1981; Sukamto,1982).

CENTRAL SULAWESI

In Central Sulawesi, Late Miocene to Recent potassic calc-alkaline magmatism occur notably along the left lateral Palu-Koro Fault Zone (Priadi et al., 1999). This granitoid is supposed to be correlated with the collision of Banggai-Sula micro-continent with Sulawesi Island in Middle Miocene but the detailed studies about its genesis and its ascent mechanism are still limited. Based on their petrological aspects, association with other rocks/formations, degree of alteration, and chemical characters, this Neogene granitoid can be classified into at least three groups, from old to young, and they demonstrate a systematical change in their features: Coarse and KF-megacrystal bearing granitoid (Granitoid-C) is distributed in the northern and southern limits of Palu-Koro areas. They can be easily recognized as they present coarse equigranular or coarse and containing KF-megacrystals. Several K-Ar age dating indicate its ages ranging from 8.39 Ma to 3.71 Ma. Two petrographic characters can be distinguished: granitoid containing biotite and hornblende as mafic minerals (4.15-3.71 Ma and 7.05-6.43 Ma), and granitoid containing biotite as major mafic mineral (8.39-7.11 Ma). Medium mylonitic-gneissic granitoids (Granitoid-B) are exposed relatively in the central areas (around Palu-Kulawi). They are all present medium grained granitoids and sometimes contain xenoliths. This granitoid can also be subdivided into hornblende-biotite and biotite bearing granitoids. The former is distributed in the southern part (Saluwa-Karangana) and dated of 5.46-4.05 Ma. Whereas the latter which is dated 3.78-3.21 Ma exposes around Kulawi. Fine and biotite-poor granitoid (Granitoid-A) represent the youngest granitoid in Palu-Koro area (3.07-1.76 Ma), they occur as small dykes cutting the other granitoids. The rocks are clear, white and containing few biotites as single mafic minerals, most of them are concentrately exposed between Sadaonta-Kulawi in the central parts. Together with these aplitic dykes are also found lamprophyric dykes (minnette type). Pre-Neogene Gneissic Granitoid (Granitoid-D) is found in certain limited areas around Toboli. Based on geological map of Sukamto et al. (1973) its distribution can be extrapolated to extend north-south in Toboli-Kasimbar areas. It mainly consists of granites that are composed of quartz, K-feldspar, plagioclase and muscovite. The occurrence of muscovite and its older age (96.37 Ma), makes this granitoid differ from the others. Laterally these granitoids present a relative circular distribution with Granitoid-A around Kulawi as the focus and rimmed by Granitoid-B and C. The oldest Granitoid-D elongates north-south at the eastern part of the concentric distribution (Figure-1).

NORTH SULAWESI

The North Sulawesi Arc, defined primarily on the basis of distribution of Lower Miocene arc-related rocks, extends for about 500 km onshore, from 121o E to 125 o 20’ E, and has a relatively constant width of 50 – 70 km, with elevations up to 2065 m. Higher elevations up to 3225 m are present at the neck of Sulawesi. The evolution of the North Sulawesi Arc may be divided into two main stages, with respect to the mid- Miocene collision of the arc with the Sula Platform: (1) west-directed subduction during the Early Miocene, and (2) post-collisional rifting and uplift of the arc, and inception of subduction along the North Sulawesi Trench during the Late Miocene to Quaternary. Geological relationships, paleontology (summarized on published 1: 250,000 maps) and preliminary K – Ar dating (Lowder and Dow 1978, Villeneuve et al. 1990, Perello 1992, Priadi, pers. commun. 1991) suggest two main periods of magmatic activity during the Neogene and Quaternary, namely, 22 – 16Ma (Early Miocene) and younger than 9 Ma (Late Miocene – Quaternary), i.e. pre- and post-collision of the arc with the Sula Platform. Pliocene and active Quaternary volcanicity belonging to the Sangihe Arc (Fig. 1) conceals much of the Early Miocene geology near Manado (Fig. 4). Small exposures of andesite and diorite below Quaternary volcanic cover on the Sangihe islands, north of Manado, suggest that older arc volcanics continue offshore, possibly to Mindanao (Fig. 1), and form the basement to the present-day Sangihe Arc. Neogene arc-related volcanic rocks are absent between Tolitoli and Palu in the neck of Sulawesi (Fig. 4), partly due to high uplift rates and deep erosion. Lower Miocene granitoids are not known, and there seems to be little evidence that the Early Miocene arc extended into the neck. Despite this, it is still inferred that the Early Miocene Benioff zone extended beneath the neck, and south to an intersection with the paleo-Palu – Matano transform fault (Fig. 1). In Western Sulawesi, south of Makale (Fig. 1), potassic alkaline (or shoshonitic) magmatism related to rifting rather than subduction was dominant during the Neogene (Yuwono et al. 1985, Leterrier et al. 1990, Priadi et al. 1991).

EASTERN SULAWESI OPHIOLITE BELT

The ophiolite complex and its pelagic sedimentary cover in the East and Southeast Arms of Sulawesi was named the Eastern Sulawesi Ophiolite Belt by Simandjuntak (1986). The belt comprises mafic and ultramafic rocks together with pelagic sedimentary rocks and melange in places. Ultramafic rocks are dominant in the Southeast Arm of Sulawesi, but mafic rocks are dominant farther north, especially along the northern coast of the East Arm (Smith, 1983; Simandjuntak, 1986). A complete ophiolite sequence was reported by Simandjuntak (1986) in the East Arm, including ultramafic and mafic rocks, pillow lavas and pelagic sedimentary rocks dominated by deep-marine limestone and bedded chert intercalations. Much of the complex is highly faulted and tectonised with blocky exposures. Based on limited geochemistry data (16 basalt samples), the Eastern Sulawesi Ophiolite Belt was probably of mid-oceanic ridge origin (Surono, 1995).

SOUTH EAST SULAWESI

The Southeast Sulawesi continental terrain occupies a large area in the Southeast Arm of Sulawesi, whereas the ophiolite belt is mainly restricted to the northern part of this arm (Fig. 2). The continental terrane, which trends northwest- southeast, is bounded by the Lawanopo Fault in the northeastern edge and by the Kolaka Fault in southwestern edge (Figs 1-2). The terrain is separated from the Buton Terrain by a thrust fault, and at the eastern end there is an older ophiolite suite thrusting over. The continental terrane comprises metamorphic basernent, with minor aplitic intrusions, Mesozoic clastic and carbonate strata, and Paleogene limestone (Fig. 2). The basement mainly consists of low-grade metamorphic rocks. The clastic sedimentary sequences consist of the Late Triassic Meluhu Formation. Paleogene limestone units include the Tamborasi Formation and Tampakura Formaticm (Figs 2, 3).

Basement

The low-grade metamorphic basement rocks form the dominant component in the Southeast Arm (Fig. 2). Tbe age of metamorphism is not clear yet. However, there are recognized an older metamorphic epidote-amphibolite kcies and a younger low grade dynamo-metamorphic glaucophane schist facies. The older metamorphism was related to burial, whereas the younger metamorphism was caused by large scale overthrusting when the Southeast Sulawesi continental terrane collided with the ophiolite belt, The metamorphic rocks were intruded by aplite and overlain by quartz-latite lava in places, especially along the western coast of Bone Gulf.

Mesozoic sedimentary rocks

In Kendari area, the basement rocks are unconformably overlain by the Late Triassic Meluhu Formation (Figs 2,3), which consists of sandstone, shale and mudstone. The Meluhu Formation composes of 3 members: from oldest to youngest they are the Toronipa, Watutaluboto and Tuetue Members. The Toronipa Member consists of meandering river deposits and is dominated by sandstone intercalated with conglomeratic sandstone, mudstone and shale. The Watutaluboto Member is a tidal-delta deposit dominated by mudstone intercalated with thin beds of sandstone and conglomerate. The Tuetue Member consists of mudstone and sandstone passing up into shallow marginal marine marl and limestone. Sandstone in the Toronipa Member consists of litharenite, sublitharenite and quartzarenite derived from a recycle orogen source The ubiquitous metamorphic rock fragments in the sandstone indicates that the source area for the Meluhu Formation was dominated by metamorphic basement. The metamorphic rocks were probably covered by a thin sedimentary succession. The small percentage of volcanic fragments in the formation suggests that volcanic rocks also formed a thin layer with limited lateral extent in the source area. The rare felsic igneous fragments were probably derived from dykes and/or si1ls that intruded the rnetamorphic basement. The Meluhu Formation is time equivalent to the Tinala Formation of the Matarombeo Terrain and the Tokala Formation in Siombok Terrain (Figs 2,4). Lithologically, these three formations are similar, with clastic-dominated sequences in their lower parts and become carbonate-dominated in the higher part of the formations. Halobia and Daonella in the Meluhu, Tinala and Tokala Formations indicate a Late Triassic age. The presence of ammonoids and pollen in the Tuetue Member of the Meluhu Formation strongly supports this interpretation. The clastic sedimentary sequence of the Tinala Formation (Fig. 4), in the Matarombeo Terrane, is successively overlain by the fine-grained clastic Masiku Formation and the carbonate-rich Tetambahu Formation. Molluscs, ammonites and belemnites are abundant in the lower part of the Tetambahu Formation and indicate a Jurassic age. The upper part of the formation contains cherty limestone and chert nodules rich in radiolarians. The radiolames suggesting a Jurassic-Early Cretaceous age. In the East Arm, the Tokala Formation of the Siombok and Banggai-Sula Terranes (Fig. 4), consists of limestone and marl with shale and chert intercalations. Steptorhynchus, Productus and Oxytoma are present in the formation that suggest a Permo-Carbonaferous age. However, Misolia and Rhynchonel1a are found within a limestone bed in the formation indicating a Late Triassic age. Due to lithological similarity between this formation and the upper Meluhu Formation, a Late Triassic age is most probable for the Tokala Formation age, while the Pamo-Carboniferous age probably represents a basement age. The Tokala Formation is overlain by the pink granitic conglomerate of the Nanaka Formatian, which may have been derived from the widespread granitic basement in the Banggai-Sula Islands. The overlying Nambo Formation consists of sandstone and shale containing common belemnites and ammonites indicating a Jurassic age.

Paleogene limestone

Paleogene limestone sequences of the Tampakura Formation (400m thick) unconformably overlie the Meluhu Formation in the Southeast Sulawesi Continental Terrane. The formation consists of oolite, lime mudstone, wackestone and locally packstone, grainstone and framestone. In the lowest part of the formation, there is a clastic strata consisting of mudstone, sandstone and conglomerate. The formation contains foraminiferas indicating a Late Eocene-Early Oligocene age. Nanoflora in the formation indicating a broad Middle Eocene to Middle Miocene age. Thus deposition of the formation must have taken place during the Late Eocene-Early Oligocene. Initial deposition was in a delta environment where siliciclastic materials were dominant. A reduction in clastic sediment supply allowed an intertidal-subtidal carbonate facies to develop extensively on a low relief platform. Carbonate buildups, dominated by coralline 6amestone, and elongate carbonate sand bodies or barriers formed a rimmed shelf that protected and enclosed the carbonate tidal flat environment and isolated it from direct marine influence. Reflux dolomitizations took place in the intertidal- supratidal zones as Mg-rich fluids moved back towards the sea The similar Paleogene carbonate sequence of the Tamborasi Formation was deposited in shallow marine environments. Based on their ages and lithologies, the Tampakura and Tamborasi Formation (probably also the Lerea Formation in the Matarombeo) were probably deposited on a single broad shallow marine shelf, The shelf surrounded an island composed of metamorphic and granitic basement and Mesozoic clastic successions (Meluhu, Tinala and Tetambahu Formations). Equivalent units in the East Arm (the Banggai-Sula Terrane) include the Eocene-Oligocene limestone of the Salodik Formation, which interfingers with marl in the Poh Formation (Figs 1, 4).

EASTERN SULAWESI

The oldest rock formation of Triassic age is called Tokala formation. This consists of limestone and marl with intercalations of shales and cherts, regarded as being deposited in a deep sea environment. Another rock facies of the same age deposited in a shallow sea is formed by Bunta formation consisting of altered fine-grained clastic sediments such as slate, metasandstone, silt, phyllite and schist. In the East Arm of Sulawesi is also found the so called Ophiolite complex of late Jurassic to Eocene age which originated from an oceanic crust (Simandjuntak, 1986). This complex is found in a tectonic contact with Mesozoic sediments and consists of mafic and ultramafic rocks such as harzburgite, lherzolite, pyroxenite, serpentinite, dunite, gabbro, diabase, basalt and microdiorite. These rocks under went several times of deformations and displacements from their original place of which the last one was of Middle Miocene age. The Tokala and Bunta formations are unconformably overlain by Nanaka formation consisting of coarse-grained well-bedded clastic sediments such as conglomerate, sandstone with intercalations of silts and coal lenses. Among the fragments within the conglomerate are found red granite, metamorhpic rocks and chert which presumably originated from the socalled banggai-sula microcontinent (Simandjuntak, 1986). The age of this formation is assumed as Lower to Middle Jurassic and it was formed in a paralic environment. Conformably overlying the Nanaka formationis found the Nambo formation of Middle to Upper Jurassic age. This shallow marine unit consists of fine clastic sediments of sandy marl and marl containing belemnite and Inoceramus. The Upper Jurassic to Upper Cretaceous Matano formation consists of limestone with intercalations of chert, marl and silt. Unconformably overlying the Nambo formation are found the Salodik and Poh formations which interfingers each other. These formations are of Eocene to Upper Miocene in age. The Salodik formation consists of limestone with intercalation of marl and sandstone containing quartz fragments. The abundance of corals, algae and larger foraminifera found in this formation suggest that it was formed in a shallow marine environment. The Salodik formation is in a fault contact with the Ophiolite Complex. The Poh formation consists ofmarl and limestone with sandstone intercalations. The foraminifera assemblage of this formation indicating an age of Oligocene to the lower part of Upper Miocene. Nanno planktons within this formation suggest Oligocene to Middle Miocene age. The Molasse of Sulawesi which consists of Tomata, bongka, Bia, Poso,Puna and Lonsio formations (Surono, 1989) is of Middle Miocene to Pliocene. The Molasse contains conglomerate, sandstone, silt, marl and limestone, deposited in paralic to shallow marine facies. It overlies unconformably the Salodik and Poh formations as well as the Ophiolite complex. The Middle Miocene to Late Pliocene Bualemo volcanics interfinger with the Lonsio formation of the Molasse and consist of pillow lava and volcanic rocks. Unconformably overlying the Molasse of Sulawesi is the Pleistocene Luwuk formation, consisting of coral reef limestone with intercalations of marl in its lower part.

SULAWESI MOLASSE

The Sulawesi Molasse was deposited after the collision between the continental fragments and the ophiofite belt. Tbe molasse is widely distributed throughout eastern Sulawesi and consists of coarse- to fine-grained clastic sequences with minor shallow marine carbonate sequences in places. The molasse in the Southeast Arm was divided into the conglomerate-dominated Alangga and Pandua Fonnations, a marl and limestone sequence of the Boepinang Formation, limestone of the Eemoiko Formation, and coarse- to fine-grained clastic strata of the Langkowala Formation. Boulders of pink granite found in the Early Miocene molasse sequences on the northern coast of the Southeast Arm and on Selabangka and Manui Islands may have been derived from the Banggai-Sula Islands. The molasse in the Southeast Arm is slightly older (Early Miocene) than in the East Arm where the collision between the Banggai-Sula continental terrane and the East Sulawesi ophiolite belt resulted in the deposition of Late Miocene molasse.

CONTINENTAL FRAGMENTS

The continental fragments in the Sulawesi region, including Central and Southeast Sulawesi, Banggai-Sula and Buton, are believed to have been derived from part of the northern Australian continent (Pigram et al., 1985; Metcalfe, 1988, 1990; Audley-Charles and Harris, 1990; Audley-Charles, 1991; Davidson, 1991; Surono, 1997). They probably broke off from the Australian continent in the Jurassic and moved northeast to their present position. Audley-Charles and Harris (1990), Metcalfe (1990) and Audley-Charles (1991) termed them allochthonous continental terranes. Metamorphic rocks are distributed widely in the eastern part of Central Sulawesi, the Southeast Arm and the island of Kabaena. The metamorphic rocks can be divided into amphibolite and epidote-amphibolite facies and a low grade dynamometamorphic group of glaucophane or blueschist facies (deRoever, 1947, 1950). The amphibolite and epidote-amphibolite facies are older than the radiolarite, ophiolite and spilitic igneous rocks which are found in the metamorphic belt of the Central Sulawesi Province, while the glaucophane schist, on the other hand, is younger. The glaucophane schist is consistent with a high pressure and low temperature petrogenesis but these rocks have only had a reconnaissance petrological examination. Glaucophane becomes more abundance westward (Sukamto, 1975b). Except in Buton, the metamorphic rocks were intruded by granitic rocks in the Permo-Triassic. In the Southeast Sulawesi, Banggai-Sula and Buton Microcontinents metamorphic rocks form the basements of the Mesozoic basins. These rocks are unconformably overlain by thick units of Mesozoic sedimentary rocks, dominated by limestone in Buton and siliciclastic rocks in the Southeast Sulawesi and Banggai-Sula Microcontinents. Paleogene limestone is found on all of the microcontinents (Smith, 1983; Surono, 1986, 1989a, b; Supandjono et al., 1986; Surono and Sukarna, 1985; Garrad et al., 1989; Soeka, 1991). In the Late Oligocene-Middle Miocene time, westward-moving slices of one or more Indonesian-Australian microcontinents collided with the ophiolite complex of East and Southeast Sulawesi. The collision produced melange and an imbricate island arc zone of Mesozoic and Paleogene sedimentary strata from the microcontinents, with overthrust slices of ophiolite (Silver et al., 1983a, b). During the collision, local sedimentary basins formed in Sulawesi. After the collision, basins became more widely developed throughout Sulawesi. Sedimentation in the Southeast Arm began earlier (Early Miocene) than in the East Arm (Late Miocene, Smith, 1983; Surono, 1989a, b). Both these sequences are commonly referred to as the Sulawesi Molasse (Sarasin and Sarasin, 1901) and consist of a major clastic succession and minor reefal limestone. Most of the molasse was deposited in a shallow marine environment but in some places it was deposited in fluvial to transitional environments (Simandjuntak et al., 1981a, b, 1984; Surono et al., 1983; Rusmana et al., 1988; Surono, 1989a, b, 1996).

BONE BASIN

Bone Basin is located between south and southeast arm of Sulawesi, interpreted as a composite basin, with its origin as a subduction complex and suture between Sundaland and Gondwana-derived microcontinents, which subsequently evolved as a submerged intramontane basin. Tectonic and stratigraphic evolutions of the Bone Basin are still poorly understood due to limited data. A new model based on surface geology, seismic and single well data is presented for the tectonic and stratigraphic evolution of Bone Basin. During Early Tertiary or older, a westward subduction complex was probably developed to the east of western Sulawesi and Bone Basin was in a fore arc setting. A collisional event occurred between Australian-derived microcontinents and the Early Tertiary accretionary complex during Middle Miocene resulting in eastward obduction of the accretionary complex during Middle Miocene resulting in eastward obduction of the accretionary complex onto the microcontinents. The westerly continental moving microcontinents then collided against and partly was subducted beneath the western Sulawesi during Late Miocene. The compression from the collision propagated a major back-thrust system westward to the subduction zone generating foldbelts as indicated by the west-verging Kalosi and Majne fold belts. The two colliding plates then were locked up during the Pliocene and the continued plate convergence was accommodated by strike-slip movements along the Walanae, Palukoro and other faults. In the southern part of Bone Basin, westerly movement of the microcontinents did not reach the collision stage with western Sulawesi. Instead, Southeast Sulawesi was rotated eastward resulting in a major extensional fault cutting along the middle of the Bone Basin (Sudarmono, 1999). Stratigraphic record is very limited as only one well was drilled in the basin. The well indicates that the northern part of the Bone Basin basically consist of two marine sedimentary packages separated by a major Pliocene unconformity, which are pre-collision and post-collision sediments. The pre-collision sediments is of Late to Lower Miocene age consisting of predominantly calcareous claystone with rare limestone beds in the upper part and a conglomeratic layer in the lowermost part. The post-collision sediment is a syn-orogeneic sequence consisting of interbedded sands and clays with a few thin sporadic lenses of lignites. The lowermost part of the package and overlying the major Pliocene unconformity is a layer of fine to coarse grain sandstones grading to conglomerates (Sudarmono, 1999).

STRUCTURAL GEOLOGY

Sulawesi Island and its surroundings is one of the most complicated active margin in term of geology, structure and tectonic as well. The region represents a center of triple junction plate convergene, due to the interaction of three major earth crusts (plates) in Neogene times (Simandjuntak, 1992). This convergence gave rise to the development of all type of structures in all scales, including subduction and coliision zone, fault and thrust and folding. At present most of the Neogene structures and some of the pre-Neogene structures are still being activated or reactivated. The major structures include Minahasa Trench, Palu-Koro Fault System and its spalys of Balantak-Sula Fault, Matano Fault, Lawanopo Fault, Kolaka Fault and Kabaena Fault, Batui Thrust, Poso Thrust and Walanae Fault.

MINAHASA TRENCH

The Minahasa Trench is surfece expression of Benioff zone, inwhich the Sulawesi Sea crust being subducted beneath the North Arm of Sulawesi in late Palogene times (Fitch, 1970; Katili, 1971; Cardwell and Isacks, 1978; Hamilton, 1979; McCaffrey et al, 1983; Simandjuntak, 1993a). The subduction seems to be culminated in Neogene comtemporaneously with the west-southwest dipping collision zone between the Eastern Sulawesi Ophiolite Belt against the Banggai-Sula Platform along the Batui Thrust in the south. Seismicity suggests that at the present the Minahasa Trench seems to be dying out (Mc Caffrey et al, 1983; Kertapati et al, 1992). Simandjuntak (1988) suggested that recently, the eastern portin of the subduction zone seems to have been reactivated and produced the Minahasa Volcanic arc.

PALU-KORO FAULT SYSTEM

The Palu-Koro Fault System for the first time is defined by Sarasin (1901), and Rutten (1927) described the fault zone stretched on nearly N-S direction for at least 300 km long in Central Sulawesi. Sudrajat (1981) described that the Palu-Koro Fault stretchs from west Palu City to the Bone Bay in the southeast for some 250 km long and calculated the transcurrent movement in the ranging of 2-3.5 mm to 14-17 mm/year. Tjia (1981) analysed the rate of up-lifting of coralline reefs within the fault zone of some 4.5 mm/year. Indriastuti (1990) calcaulated the means of horizontal maovement of 1.23 mm/year. Bemmelen (1970) and Katili (1978) suggested that the northern portion of the fault system is dominated by vertical movement whereas the southern part is dominated by sinistral wrench movement. Walpersdorf et al (1997) on the basis of interferometric GPS analysis found out that the sinistral wrench movement of the Palu-Koro Fault System on te rate of 3.4 mm/year. Seismicity shows that at the present the Palu-Koro Fault being at least segmently reactivated (Kertapati et al, 1992; Soehaemi and Firdaus, 1995 ). Simandjuntak (1993a, b) thought that the Palu-Koro Fault System continued to Bone Bay, cut across the Flores Thrust and terminated in the Timor Trough in the south and to the north is termianted in Minahasa Trench. He also pointed out that during the history of fault movement, the Palu-Koro Fault was dominated by a sinistral transpressional movement, giving rise to the up-lifting of the mountain ranges along the fault zone. Althought in recent time the fault system was subjected to a transtensional sinistral wrenching causing the development of graben like basins such as Palu Valley and small lakes in many parts along the fault zone. He also further suggested that the development of Bone Bay was magnified by the sinistral transtensioanl movement of Palu-Koro Fault System in very late Neogene time. The Palu-Koro Fault System in Sulawesi is connected with Sorong Fault System in Irian Jawa via Balantak-Sula Fault, Matano-South Buru Fault. To the south the Palu-Koro Fault merges with the Lawanopo Fault. Kolaka Fault and Kabaena Fault (Simandjuntak, 1993a)..

BATUI THRUST

Simandjuntak (1993a) defined that the Batui Thrust is surface expression of the collision zone between Banggai-Sula Platform against Eastern Sulawesi Opiolite Belt in Neogene time. The thrust bounds the ophiolite belt in the hanging wall from the micro-continents in the foot wall regims. The thrust can be obsrved clearly on the landsat imagery of the region (Hamilton, 1979). The thrust strechts from Balantak in the eastern tip of the East Arm of Sulawesi to the SW in Morowali, Tomori Bay. The thrust is disrupted and cut across by a number strike-slip fault, Toili Fault, Ampana Fault and Wekuli Fault. Its continuation further to the south in central, Southeast Arm, Buton and Kabaena Islands seems to have been greatly disrupted and modified by post-collision faults and hence it can not be traced as a continuous thrust zone. Seismicity shows that at present the thrust might be reactivated (McCaffrey et al, 1983; Kertapati et al, 1992). The occurrence of at least three terraces of Quaternary coraline reefs along the southern coast of the East Arm of Sulawesi also testifies the recent reactivation of the thrust (Simandjuntak, 1986, 1993a).

POSO THRUST

Poso Thrust is defined as structural contact zone between the Central Sulawesi Metamorphic Belt (CSMB) and the Western Sulawesi Magmatic Belt (Bemelen, 1949; Hamilton, 1979; Simandjuntak et al, 1991; Simandjuntak et al, 1992). The thrust is believed to have instrumented the up-thrusting of high pressure metamorphics (CSMB) from the depth in Benioff zone on to the top of magmatic belt in Neogene times. Seismicity suggests that at the present the thrust is no longer active (Kertapati et al, 1992). However, the recent earth quake in the west coast of Tomini Bay indicates that at least the northern portion of the thrust being reactivated.

WALANAE FAULT

The Walanae Fault is defined as a sinistral wrench faulting trending in NW-SE direction, cut across the South Arm of Sulawesi. The fault seems to be continued further to the northwest cut across Makassar Strait and merged with the Phaternoster-Lupar suture in Kalimantan and to the south is terminated in the Flores Thrust. In Quaternary the fault seems to have been reactivated transtensionally causing the development of Walanae Depression. Seismicity suggests that at the present the fault is no longer active or dying out.

NOTES ON THE MAKASSAR STRAIT

Katili (1978) suggested that the Makassar Strait was tectonically developed due to the rifting of the region with axis trending nearly N-S direction parallel to the long axis of the strait. Situmorang (1983), on the basis of seismic reflection profile across Makassar Strait found out that no a new developing oceanic crust beneath of the Tertiary sequences at the sea floor of the strait. He further suggested that the basement of the strait is more likely of continental crust. The occurrence of the Neogene fissured volcanics in and along the Lupar-Phaternoster suture and other parts in the interior of Kalimantan (Bergman et al, 1988; Harahap, 1996; Hutchison, 1996; Simandjuntak, 1999) and a similar sosonitic volcanics in wsetern South Arm of Sulawesi (Pryadi, 199 ) suggest the development of extensional tectonic in the region on Neogene times. The development of Makassar Strait more likely being related with the extensional tectonic occurring in many parts of central Indonesia in Neogene times.

TECTONIC DEVELOPMENT OF SULAWESI

The peculiar ‘K’ shaped of Sulawesi Island may indicates the complexity of geology and tectonics of the region. On the basis of data obtained on geology and geophysics Simandjuntak (1993) summarized the tectonic evolution of Sulawesi and its surroundings, which is related with the (re)occurrence of a number types of tectonism, including a) Cretaceous Cordileran type subduction, b) Mesozoic tectonic divergence, c) Neogene Tethyan type collision and d) Quaternary double opposing collision.

CRETACEOUS CORDILERAN TYPE SUBDUCTION

A Cretaceous Cordileran type subduction is recorded by the development of a west-dipping Benioff zone in and along western Sulawesi, inwhich the proto- Banda Sea crust subducted beneath south-southest margins of Sunda Shield (SE Eurasian Craton). The occurrence of Late Cretaceous high pressure metamorphic rocks in the Central Sulawesi Metamorphic Belt, the Cretaceous-Paleogene melange wedges associated with metamorphics and ophiolitic rocks, the Paleogene volcanics in the Westren Sulawesi Magmatic Belt and the ophiolites in the Eastern Sulawesi Ophiolite Belt are thought to have been developed during and subsequent to this subduction (Simandjuntak, 1980). The presence of Late Cretaceous-Paleogene flysch sediments associated with basaltic lavas may represent an upper trench slope sequences during this palte convergence.

MESOZOIC TECTONIC DIVERGENCE

Meanwhile, further to the south-southeast, subsequently after the Permo-Triassic thermal doming the northern continental margins of Australia were rifted due essentially to the extensional tectonic. The continental fragments, then were detached and displaced north-northwestwards to form the present micro-continents in the Banda Sea region (Pigram & Panggabean, 1984), including the Banggai-Sula Paltform, Tukangbesi-Buton Platform and Mekongga Platform (Simandjuntak, 1986), During the history of the detacheement and northwestwards displacement, the continental blocks were fragmented to form those micro-continents occurring in the Banda Sea region. And by the Neogene times, some of the micro-continents were collided with the subduction complex and ophiolite belt in the western margin of Banda Sea region. The tectonic divergence seems to be essentially dominated by a transcurrent-transformal displacement along the line of Sorong Fault System together with its splays of steep faults in the region (Simandjuntak, 1986, 1993).

NEOGENE TETHYAN TYPE COLLISION

The north-northwestwards moving continental fragments (micro-continents) of Banggai-Sula Platform, Tukangbesi-Buton Platform and Mekongga Platform collided with the subduction complex (CSMB) and the ophiolite belt (ESOB) in Neogene times. This tectonic convergence is typically Tethyan collision inwhich the the platforms underplated the ophiolite belt and subduction complex. At present the collision zone is marked by the occurrence of Neogene melange wedges in places along the Batui Thrust in the East Arm of Sulawesi (Simandjuntak, 1986). The collision characteristically produced no volcanic arc and geometrically without the development of fore-arc and back-arc basinal setting (Simandjuntak, 1988). The end products of this collision is characteristicaaly marked by the obduct-ing (up-thrusting) of the ophiolite suite onto the margins of the micro-continents and the thrusting-up of the subduction complex (CSMB) over the Western Sulawesi magmatic arcs (Simandjuntak, 1991; Bergman et al, 1996). The Papua New Guinea Ophiolite Belt is also emplaced by an obduction tectonics (Davies, 1976). During the end and subsequent to this collision, the deposition of post-orogenic coarse clatics of mostly molasse type sediments took place in the Late Neogene times. The molasses are mostly marine, but partly are terrestrials as indicated by the occurrence of lensoidal lignites, which seems to have been acummulated in an isolated and fault-bounded graben like basins especially in the interior of Central Sulawesi. The marine molasse at least partly seem to have been deposited in a submarine fan environmental setting.

QUATERNARY DOUBLE OPPOSING COLLISION

At present an active volcanics in and along the Minahasa-Sangihe Volcanic Arc appears to have been initiated by the development of a double- opposing subduction in northern Sulawesi in Neogene and reactivated in Quaternary. The plate convergence is marked by the development of south-southeastwards-dipping subducted crust of Sulawesi Sea beneath the North Arm of Sulawesi couples with the westward-dipping subducted crust of Maluku Sea in the north with its southern continuation along the Batui Thrust, inwhich the Banggai-Sula Platform underpalted the Eastern Sulawesi Ophiolite Belt in the East Arm of Sulawesi (Simandjuntak, 1991). On the basis micro-seismicity analysis McCaffrey et al (1983) suggest that the southern collision might be (re) activated at the present time. The occurrence of at least three terraces of Quaternary reefal limestones in and along the southern coast of the East Arm of Sulawesi testifies the reactivati-on of thie plate convergence and the rapid uplifting of the region.

Where To Look For Placers Gold

Placers can be found in virtually any area where gold occurs in hard rock (lode) deposits. The gold is released by weathering and stream or glacier action, carried by gravity and hydraulic action to some favorable point of deposition, and concentrated in the process. Usually the gold does not travel very far from the source, so knowledge of the location of the lode deposits is useful. gold also can be associated with copper and may form placers in the vicinity of copper deposits, although this occurs less frequently.

Geological events such as uplift and subsidence may cause prolonged and repeated cycles of erosion and concentration, and where these processes have taken placer deposits may be enriched. Ancient river channels and certain river bench deposits are examples of gold-bearing gravels that have been subjected to a number of such events, followed by at least partial concealment by other deposits, including volcanic materials.

Residual placer deposits formed in the immediate vicinity of source rocks are usually not the most productive, although exceptions occur where veins supplying the gold were unusually rich. Reworking of gold-bearing materials by stream action leads to the concentrations necessary for exploitation. In desert areas deposits may result from sudden flooding and outwash of intermittent streams.

As material gradually washes off the slopes and into streams, it becomes sorted or stratified, and gold concentrates in so-called pay streaks with other heavy minerals, among which magnetite (black, heavy, and magnetic) is almost invariably present. The gold may not be entirely liberated from the original rock but may still have the white-to-gray vein quartz or other rock material attached to or enclosing it. As gold moves downstream, it is gradually freed from the accompanying rock and flattened by the incessant pounding of gravel. Eventually it will become flakes and tiny particles as the flattened pieces break up.

Some gold is not readily distinguishable by the normal qualities of orange-yellow to light yellow metallic color and high malleability, where it occurs in a combined form with another element, such as tellurium. Upon weathering, such gold may be coated with a crust, such as iron oxide, and have a rusty appearance. This "rusty gold," which resists amalgamation with mercury, may be overlooked or lost by careless handling in placer operations.

As mentioned before, the richest placers are not necessarily those occurring close to the source. Much depends on how the placer materials were reworked by natural forces. Streambed placers are the most important kind of deposit for the small-scale operator, but the gravel terraces and benches above the streams and the ancient river channels (often concealed by later deposits) are potential sources of gold. Other types of placers include those in outwash areas of streams where they enter other streams or lakes, those at the foot of mountainous areas or in regions where streams enter into broader valleys, or those along the ocean front where beach deposits may form by the sorting action of waves and tidal currents. In desert areas, placers may be present along arroyos or gulches, or in outwash fans or cones below narrow canyons.

Because gold is relatively heavy, it tends to be found close to bedrock, unless intercepted by layers of clay or compacted silts, and it often works its way into cracks in the bedrock itself. Where the surface of the bedrock is highly irregular, the distribution of gold will be spotty, but a natural rifflelike surface favors accumulation. Gold will collect at the head or foot of a stream bar or on curves of streams where the current is slowed or where the stream gradient is reduced. Pockets behind boulders or other obstructions and even moss-covered sections of banks can be places of deposition. Best results usually come from materials taken just above bedrock. The black sands that accumulate with gold are an excellent indicator of where to look.

It should be kept in mind that each year a certain amount of gold is washed down and redeposited during the spring runoffs, so it can be productive to rework some deposits periodically. This applies chiefly to the near-surface materials such as those deposited on the stream bars or in sharp depressions in the channels. The upstream ends of stream bars are particularly good places for such deposits. Where high water has washed across the surface by the shortest route, as across the inside of a bend, enrichment often occurs.

A rifflelike surface here will enhance the possibility of gold concentration. In prospecting areas with a history of mining, try to find places where mechanized mining had to stop because of an inability to follow and mine erratic portions of rich pay streaks without great dilution from nonpaying material. Smaller scale selective mining may still be practical here if a miner is diligent.

Gold Prospect in Bombana, Southeast Sulawesi

A gold panning boom occurred mid 2008 in Bombana, Southeast Sulawesi, marked by the discovery of nuggets by the local community in areas surrounding Tahi Ite. This discovery led to the massive influx of gold panners, both from the local communities and outside Bomabana, even from outside Sulawesi.

Apart from its positive impacts on the local community, gold alluvial panning by the people in Rarowatu and North Rarowatu districts, Bomabana will also create social, administrative, technical, and environmental issues.

The exploding amount of gold opportunists will eventually create social issues concerning land ownership and use of road access, environmental issues which includes rapid deforestation, and administrative issues concerning permits and regional income.

For these reasons alone, gold panning in the area must be controlled and managed correctly. Gold prospecting activities carried out by a group from the Mineral Research Program Group, of the Geological Body aims to map out the locations of gold deposits in the area, which may then be used as a technical reference for the local government in managing the area and issuing permits.

Prospecting activities is carried out by geological mapping methods, alluvial sediment mapping, geochemical and heavy mineral concentrate sampling. The prospected areas include the districts of North Poleang, Rarowatu, and North Rarowatu, which are all located within the regency of Bombana, Southeast Sulawesi.

Prospect Outcome

As many as 20 active river sediment samples, 73 concentrates, and 20 rock samples were examined in laboratories for their geochemical, mineralogy, petrography, and mineragraphy properties. The concentrates observed contained traces of gold ranging from fine to coarse sediment traces. These gold sediments are found in oxidized schist rocks and alluviums in loose materials, clays, and overburden.

After researching a total area of 31.790 Ha, the prospecting team has found that there are two types of gold sediments, primary sediment which is found in schist rocks (oxidation) and secondary sediments in alluvium areas. The minimum and maximum amounts of gold discovered ranges from 0,16405 g/m³ to 22,12 g/m³ respectively, with a minimum amount of secondary gold at 196,53 kg and a maximum of 26,499,76 kg (equivalent to 26,50 tons).

Alluvial gold deposits in the area may be exploited by local communities through a local panning process in rivers which originated from Tangkeno Wumbubangka hill. Alluvium gold sediments in oxidized schist rocks may also be exploited through a medium-scale mining process by means of mechanical equipments, while preserving the environment.

Detailed geological mapping is required in order to observe the spread of modified schist rocks, which is the main source of primary gold minerals. (KO/TC)

Gold Prospecting

INTRODUCTION

Gold prospectors have won many fortunes and there are many smaller finds that have gone undocumented. Here is a general introduction plus a few tips for gold prospectors or would be gold prospectors.

EQUIPMENT

The equipment available for a prospector is varied. This includes metal detectors, dryblowers and hydraulic concentrators of various shapes and sizes. While metal detectors remain the most popular tool, knowledge of other equipment is useful, especially if the prospector wants to expand his activities. Some of the basic types of equipment are described here.

Accessories

Useful accessories include the geological pick, prospecting pick, compass, times ten hand lens, safety glasses, pen knife, sample bottles and bags, hand auger and gold pan. A geological pick can be used to dig holes and split rocks while metal detecting, or collect rock samples for identification or analysis. Safety glasses are used to protect the eyes when sampling or splitting rocks. A compass is necessary when prospecting away from known tracks and landmarks. A hand lens is useful for examining fine gold and minerals. The hardness of a mineral can be tested using a pen knife. A stainless steel pen knife has a hardness of 6 1/2 on Moh's hardness scale (1-10). Sample bottles and bags are used to store fine gold and samples. They should be labelled and a list written up so locations won't be forgotten. For sampling alluvium and soils, a barrel type hand auger is useful. A gold pan is used to separate fine gold from concentrates. A pencil magnet is used to test for magnetic minerals.

Metal Detectors

V.L.F. detectors with ground balancing are the best type of detectors for prospecting. The new models of prospecting detectors have better depth and sensitivity than many of the old types. They are most useful for detecting small nuggets, which would have been missed by the old detectors. Garret, Minelab and Whites prospecting models are popular. Garret detectors have had widespread success on WA's goldfields.

Metal detectors will respond to any type of conductive or magnetic material. The metal detectors transmitting coil produces a primary electromagnetic field. When a conductive object encounters a primary electromagnetic field, currents flow through the surface of the object, called eddy currents, each producing its own secondary electromagnetic field. The secondary electromagnetic fields distort the primary electromagnetic field. A receiving coil within the metal detector receives the distorted primary electromagnetic field signal, ultimately producing an audio signal in response to the strengths of the secondary electromagnetic fields. Materials of greater conductivity produce larger and stronger secondary electromagnetic fields, and therefore audio signals, than smaller objects. For a given conductivity, the sizes and strengths of the secondary electromagnetic fields are controlled by the surface area facing the primary electromagnetic field rather than density or mass of the object. Larger surface areas produce larger fields and responses.

Manufacturers of metal detectors classify targets as either metal or mineral. Metal targets include all conductive, non-ferrous metals. Examples are silver, gold, copper and aluminium. Mineral targets consist of ferrous metals, magnetic minerals and conductive ground minerals. Examples of mineral targets are steel, iron, magnetite, iron oxide ground minerals and wet salt. These have lower conductivities than most metal targets. "Hot rocks", often encountered in the field, are concentrated forms of conductive iron oxide.

The audio signal produced usually varies according to the type of target. Gold tends to produce short, sharp signals while ferrous objects produce broad signals. A double blip will be produced on long thin objects, such as wire or nails. It is wise to do a bench test of different objects to determine their different responses. Some objects that can be used are a nail, silver coin and gold ring. With field experience, audio response from various targets, including "hot rocks", will become familiar so that identification will be easier.

Ground conditions affect the operation of metal detectors. Heavily mineralised and dense ground conditions cause the primary electromagnetic field to compress, resulting in loss of depth. Wet ground allows greater penetration of the primary magnetic field, providing better depth. Magnetic and conductive minerals (mostly iron oxide minerals) in ground soil produce background signals that can mask target objects. The ground cancel is used to decrease the effect of minerals in mineralised areas.

The following steps should be followed when tuning a manual ground cancelling detector; such as a Garret or Whites prospecting metal detector: 1. Switch on and allow to stand for 10-15 minutes to stabilise batteries. 2. Check battery condition. Batteries must be in good shape for the metal detector to work properly. 3. Place tuning on automatic. 4. Place into V.L.F. mode. The V.L.F., or ground cancelling mode, should always be used for prospecting. 5. To begin with, sensitivity should be placed on half. It can be increased or decreased according to ground conditions. If it is placed on minimum, small nuggets will be missed, therefore, always place it on the maximum allowable setting. 6. Discrimination should always be at zero. 7. Adjust tuning audio so that a humming sound is barely audible. 8. Compensate for ground conditions. To compensate for ground conditions, raise and lower the search coil (from 60cm high to 15cm low). As the search coil is lowered, the audio signal will either increase or decrease in strength. If the ground cancel knob is in its midway position then it can be turned backwards or forwards to compensate for the increase or decrease in signal strength. The ground cancel knob should be moved one complete turn each time. When the audio signal remains constant as the searchcoil is raised and lowered, the ground has been compensated for. The ground cancel will have to be adjusted as ground conditions change.

Headphones should always be worn when prospecting otherwise the batteries will drain quickly. Signals are also easier to comprehend with headphones. Some prospectors have an audio boost fitted to amplify small signals, while also suppressing very loud signals. Some audio boosts also operate with a more sensitive tone. They definitely make it easier to detect small signals. Hipmounts reduce strain when detecting for long periods or using large coils.

Dryblowers

Dryblowers are mainly used to recover fine gold in areas where there are no water supplies. Nuggets are more efficiently located by using metal detectors.

A dryblower consists of a hopper/classifier overlying an inclined riffle tray fitted with an air blower. Today's dryblowers are motorised. They range in size from small, easily portable units, less than half a metre high, to large units that can process 20 tonnes of material per hour or more.

Dry material is fed into the vibrating hopper/classifier which removes the coarsest material. All material small enough to pass through the classifier falls into a riffle tray underneath. The heavy fraction is separated using a combination of vibration and air being blown upwards to remove the dust. Finally, heavy concentrates are removed by lifting up the riffles and sweeping the concentrates into a pan. Gold is recovered by panning the heavy concentrates.

Vibrostatic dryblowers use a combination of static electric charge, air flow and vibration to collect gold. They only differ in that the gold is precharged and later attracted to assist in retaining the gold in the riffle box. This allows damp material to be processed.

Dryshakers consist of a hopper/classifier overlying an inclined riffle tray. Dry material is placed into the hopper /classifier which removes the coarse material using vibration. All fine material passes into the underlying riffle tray. High frequency, short vibrations displace light material over the riffles and out of the tray. Gold is retained by the riffles. Air blowing is not utilised by dryshakers.

Hydraulic Concentrators

When a water supply, such as a bore or stream, is available hydraulic concentrators are used to recover gold. Water can be recycled in dry areas to reduce consumption. Modern hydraulic concentrators are driven by petrol engines. The simplest of these is the rocker cradle. It consists of a hopper over a tray fitted with riffles, all mounted on a rocker.

Washdirt is fed into the hopper, the base of which contains small holes to prevent pebbles and boulders from passing through. The discarded coarse material should be examined for nuggets. Water is poured into the hopper and carries the fine material onto the riffle tray and over the riffles. At the same time, the cradle is rocked. Eddy currents form behind each riffle, the decrease in current velocity trapping heavy minerals. Matting covers the base of the riffle tray to help trap heavy minerals. Most gold will be trapped behind the first few riffles. The angle of decline of the riffle trays must be adjusted according to water flow and the amount and type of sediment. Too steep a decline will result in gold being washed away. A decline that is too shallow will have the riffles becoming choked in sand, preventing settling of gold. When the matting behind the riffles fills up, the riffles can be lifted and the matting removed. Finally, the heavy concentrates should be panned to remove any gold.

Various types of hydraulic concentrators; such as, gold screws, knelson concentrators, jigs and shaking tables can be used when large amounts of washdirt are to be treated. It should be noted that all clay material must be thoroughly dissaggregated before processing to prevent the formation of clay balls. Puddling and log washing machines are specially designed for this purpose.

Gold Pans

Gold pans are made from metal or plastic. Plastic pans are easier to maintain and just as efficient as metal pans. The pan should be large (about 40cm in diameter) and contain riffles along its side to help trap gold. A pan with riffles along one half of its side is preferable to a pan with riffles along its full circumference. This allows easy collection of gold and concentrates after panning. Metal pans are often greased and should be degreased by holding over an open flame or washing in hot, soapy water.

To use a goldpan, a layer of washdirt 3/4 inch thick is placed over the base of the pan. Rest the pan in water and rake fingers back and forth to loosen and separate material. Tilt the pan and rake coarse material to the top end, letting the fines fall back. Remove this coarse material. Next, shake the pan from side to side to help the heavy minerals settle on the bottom of the pan. Repeat these two steps four or five times. Now, place the pan in water and tilt so the fine material accumulates just under the pans edge. Remove from water and tilt back, allowing a wave to form. Tip forward again, letting the wave travel forward to carry the top material out of the pan. Next, shake the pan from side to side again. Place in water and tilt, so the light material remains just under the pans edge. Remove the pan from water and tilt back, then forward, resulting in a wave carrying the top material out of the pan. The previous few steps should be repeated until only a tablespoon of fine material is left. A gentler wash action is required as the amount of wash dirt remaining decreases. Finally, swirl the remaining washdirt on the base of the pan so contents fan out and gold specks will be visible. Gold specks can be collected with a damp finger and placed in a sample bottle filled with water. A teaspoon is useful for collecting gold when large amounts are present.

Black, magnetic sands can be removed using a magnet. Place the magnet in a plastic bag so that black sands are collected on the outside of the bag. Now, the magnet can be removed and the black sands will fall away. This prevents a buildup of black sands on the magnet. A sieve is useful to initially separate coarse material from the fine fraction. Automatic gold pans, or concentrating wheels, are an alternative to manual pans for separating gold from fine alluvium and concentrates. The wheel contains riffles which pass from the pan's edge to its centre. It is set at an angle so that washdirt remains at the lower edge of the wheel. Water is added to the centre of the wheel by a jet spray. The circular motion and spiral action of the wheel cause gold grains to migrate towards the centre of the wheel where they pass through a hole to a collecting bottle underneath. They have electric 12v motors which operate from batteries.

A new type of hydraulic concentrator, called the mini gold concentrator is replacing conventional gold pans for treating small samples of alluvium, eluvium and colluvium. The concentrator can treat twenty panloads of washdirt in the same time an expert can wash a single panload using a conventional pan. By following simple instructions a beginner can easily master the separation of gold from a shovelful of washdirt.

The unit consists of a removable dish with sieve resting on top of a lower settling pan, all clamped inside of a twenty litre bucket. To use, fill the bucket with water. Place the entire assembly into the bucket and fasten with wingnuts. Add washdirt to the upper dish and agitate the dish in a circular motion. All large material is retained in the upper bowl by the sieve. This material is discarded by removing the upper dish and sieve. Small material passes through the sieve into the lower settling pan. Agitation washes the light material over the top of the settling pan with the help of agitation blades and a helical scraper blade. Gold and heavy minerals settle to the base of the retention bowl where they remain until panning is finished. Panning continues until the bucket fills with the discarded washdirt. Finally, the unit is removed by undoing the wingnut fasteners and the bucket is emptied then refilled to start the process over again. Any gold in the retention is removed and placed in a sample bottle. To recycle water, the entire bucket with concentrator is placed in a large container so overflowing water is collected until ready for reuse. Small in size and weighing only 3.5 kg it is easily transported.

Sample Mill

The sample mill is used to crush rock samples before testing for gold. It is powered by a petrol motor for portability. A sample is placed in the hopper which feeds the pulverisers, reducing the sample to powder. Ideally, the sample mill should be adjustable so that the desired grain size can be obtained. Most sample mills have hardened steel jaws. These can produce fine steel filings that show up in the residue when panned. When more than one sample is processed, residue from previous samples carries through. Therefore, a gold bearing sample followed by a barren sample will give positive gold results in the barren sample. When accurate results are required, the mill can be cleaned by grinding quartz between samples (sometimes, particularly with ironstone's, this is not effective).

A cheaper alternative to the sample mill is the dolly pot. A dolly pot consists of two parts: a mortar and a pestle, both of large dimensions (eg. 1 litre). It is used for crushing hand samples. Samples are broken into small pieces with a hammer, then placed in the dolly pot for crushing.

Analytical Instruments

Today, the options available to the prospector for analyzing rock and mineral samples are numerous and sophisticated. Depending on the results required, techniques such as polarized light and electron microscopy; x-ray diffraction; and chemical analysis using various spectrometric methods are available.

Polarizing microscopy is the best method for identifying and examining most rocks and minerals. By observing a section of a rock or mineral with a polarizing microscope the texture, structure and mineralogy of the sample can be determined. From this information an identification can be made and the origin determined. This information is of use during mining and prospecting. For routine use, lower cost alternatives are stereo microscopes or high power pocket microscopes.

For analyzing the composition of individual minerals emission spectroscopy (ICP) or electron microscope (microprobe) analysis is carried out. Ores containing submicroscopic gold particles within their crystal lattice are analyzed with a microprobe to determine which ores are the gold carriers and where the gold is sited.

Chemical analysis of a rock or mineral sample for gold is called assaying. For most prospectors, a low cost, moderately sensitive technique is adequate. For most gold bearing samples requiring accurate determination of the gold content fire assaying is the most common method but not necessarily the cheapest. Modern fire assaying techniques can determine grades as low as 1g/tonne and starts at prices of about $12.00 per sample. In samples containing minute trace amounts of gold, more sophisticated methods are preferred.

For the geochemical explorationist who is searching for trace amounts of gold, indicating the presence of a hidden orebody, the latest analytical techniques are almost mandatory. Atomic absorption spectrometry (AAS) , induced coupled plasma (ICP) and even mass spectrometry have detection limits in the parts per billion or less and are the preferred choice. Analytical costs are higher for these methods although bulk sampling and multi-element analysis bring the costs down.

GOLD AND ITS ORES

A mineral profitably mined for its metal content is called an ore mineral, whether it is an element, such as gold, or a compound of two or more elements, such as the sulphides and tellurides. A knowledge of the properties of gold and its ores is necessary for correct identification. This information is also necessary for selecting and controlling the mining and ore processing equipment. Visual examination of a sample is usually sufficient to reduce the number of possible identities to a few, if not a single identity. Gold is most commonly found in its elemental form, with varying amounts of silver, copper and iron as impurities but also occurs in ores; such as, the sulphides and tellurides.

Beginners sometimes experience problems when identifying gold, most commonly confusing with similar minerals; such as pyrite, chalcopyrite, pyrrhotite, pentlandite and gold coloured mica. With experience, there should be no difficulty identifying gold except when it is extremely fine grained or microcrystalline. In these situations, gold cannot be easily observed and requires examination with a microscope.

The most distinctive properties of gold are its gold-yellow colour, metallic lustre, softness, high specific gravity and gold-yellow streak. Other minerals with a similar colour and lustre are often mistaken for gold. Pyrite, chalcopyrite, pyrrhotite, pentlandite and gold coloured mica are the minerals usually mistaken for gold. By keeping in mind the properties of gold each of these minerals can be eliminated. Gold is the only mineral that will easily scratch, leaving a residue of gold-yellow powder. Gold is malleable while the rest are brittle, will break and flake when struck with a hammer. When fine and placed in a pan of water, gold will sink rapidly and refuse to move, the rest will sink slowly and swirl easily. Gold occurs in grains whereas mica is flaky.

Gold also occurs as microscopic and submicroscopic particles within sulphide minerals; particularly pyrite, chalcopyrite, arsenopyrite and pyrrhotite. All of these are common within veins and zones of hydrothermal alteration and replacement. They occur as macroscopic and microcrystalline grains and crystals.

Pyrite is brass-yellow in colour with a metallic lustre and greenish-black streak. Often, it forms perfect isometric crystals in cubic or polyhedral form.

Chalcopyrite is also brass-yellow with a metallic lustre and greenish-black streak. It is easily confused with pyrite but forms tetragonal crystals instead of isometric cubes and polyhedrons. When exposed to air it often tarnishes to iridescent or deep blue. In some situations, a chemical test for copper using concentrated nitric acid may be necessary to distinguish it from pyrite.

Arsenopyrite is silver-white to steel grey with a metallic lustre and greyish-black streak. When crystalline, it exhibits monoclinic crystals usually in prismatic form. When struck with a hammer arsenopyrite often gives off a garlic smell.

Pyrrhotite is brass-yellow or brownish-bronze with a metallic lustre, greyish-black streak and orthorhombic crystals. Pyrrhotite is easily identified using a pencil magnet as it is distinctively magnetic.

Gold also occurs in compounds of gold and/or silver with tellurium. The tellurides, sylvanite and calaverite are mined for their gold content. They are quite rare, however, have been mined in Kalgoorlie as ores of gold.

Calaverite is brass-yellow to silver-white with a metallic lustre, yellowish to greenish grey streak and monoclinic crystals that are often striated.

Sylvanite is silver-white to steel grey with a metallic lustre, black streak and monoclinic crystals. The hardness of calaverite is 1 1/2 to 2 and of sylvanite 2 1/2 to 3.

GOLD Au
Colour: Gold yellow to pale yellow
Lustre: Metallic
Hardness: 2.5 to 3
Specific Gravity: 19.3 to 15.6
Fracture: Ductile and malleable
Streak: Gold yellow

Best Field Characteristics: Gold yellow colour, high SG, gold yellow streak, softness.
Similar Minerals: Pyrite and chalcopyrite have a greenish-black streak, will sink slowly and swirl in a pan of water when fine whereas gold will sink rapidly and refuse to move. They are brittle: will break and flake when touched with a knife but won't scratch. Gold is malleable and will scratch easily. Once gold has been seen and held, future identification will be simple.

Gold also occurs as submicroscopic particles within sulphide minerals, particularly pyrite, chalcopyrite, arsenopyrite and pyrrhotite. All of these are common within veins and zones of hydrothermal alteration and replacement. They occur as macroscopic and microcrystalline grains.

Pyrite is an iron disulphide.
PYRITE FeS2
Colour: Brass yellow
Lustre: Metallic
Hardness: 6 to 6.5
Specific Gravity: 4.9 to 5.2
Fracture: Uneven/brittle
Streak: Greenish-black
Crystals: Isometric, in cubes and pyritohedrons. Also occurs massive and in anhedral grains.
Best Field Characteristics: Colour, streak and cubic crystal form.

Chalcopyrite is a copper iron sulphide.
CHALCOPYRITE CuFeS2
Colour: Brass yellow
Lustre: Metallic
Hardness: 3.5 to 4
Specific Gravity: 4.1 to 4.3
Fracture: Uneven/brittle
Streak: Greenish-black
Crystals: Tetragonal, usually massive and in anhedral grains.
Best Field Characteristics: Colour and streak


Arsenopyrite is an iron arsenide sulphide.
ARSENOPYRITE FeAsS
Colour: Silver white to steel grey
Lustre: Metallic
Hardness: 5.5 to 6
Specific Gravity: 6 to 6.2
Fracture: Uneven/brittle
Streak: Greyish-black
Crystals: Monoclinic prismatic. Also massive and in anhedral grains.
Best Field Characteristics: Colour and crystals.

Pyrrhotite is an iron sulphide with small amounts of nickel and cobalt.
PYRRHOTITE Fe1-xS
Colour: Yellowish to brownish bronze
Lustre: Metallic
Hardness: 3.5 to 4
Specific Gravity: 4.6
Fracture: Uneven/brittle
Streak: Dark greyish-black
Crystals: Orthorhombic, also massive and anhedral grains.
Best Field Characteristics: Pyrrhotite is magnetic.

The tellurides are compounds of gold and/or silver with tellurium. The tellurides, sylvanite and calaverite are mined for their gold content. Calaverite is a ditelluride of gold. Sylvanite is a telluride of gold and silver. These are not common.
CALAVERITE AuTe2
Colour: Brass yellow to silver white
Lustre: Metallic
Hardness: 2.5 to 3
Specific Gravity: 9.1 to 9.4
Fracture: Uneven/brittle
Streak: Yellowish grey
Crystals: Monoclinic prismatic with striations. Also in anhedral grains.
Best Field Characteristics: Streak and striated crystals.

SYLVANITE AuAgTe4
Colour: Silver white to steel grey
Lustre: Metallic
Hardness: 1.5 to 2
Specific Gravity: 8.2
Fracture: Uneven/brittle
Streak: Black
Crystals: Monoclinic prismatic. Also in anhedral grains.
Best Field Characteristics: Hardness and streak.

Gold can be described according to its natural size and nature of occurrence. Based on these, gold occurs in six main forms:

(1) Large pieces of free gold >2mm in size that are known as nuggets.
(2) Pieces of gold and gangue (quartz, ironstone etc.) known as specimens.
(3) Coarse to fine grains of free gold 2mm to 150 microns that are visible to the naked eye.
(4) Microcrystalline gold 150 to 0.8 microns in size only visible with a microscope.
(5) Submicrocrystalline particles of gold that occur in the crystal lattice of certain sulphide ores.
(6) In compounds with tellurium.

All types show various degrees of crystallinity from rounded grains (eg. alluvial) with no crystal faces through subhedral grains with some crystal faces (hydrothermal) to crystalline grains with well developed crystal faces (hydrothermal and supergene gold). In most situations, gold is found in rounded forms, however, where open space crystallisation has occurred, such as in supergene environments, crystalline gold is common.

Nuggets are well known to metal detector operators. While many nuggets are almost pure gold, impurities of iron and quartz are common. Nuggets that have been chemically deposited or altered in the weathering profile are often intergrown with ironstone.

Large grains and veinlets of gold intergrown with quartz are derived from quartz reefs and lodes and are referred to as specimens. These are also well known to metal detector operators.

Free grains of gold that are visible to the naked eye are either intergrown with gangue in primary deposits or as loose grains within secondary deposits. Machinery is required to separate gold grains from unwanted gangue. Fortunately, the high specific gravity of gold enables it to be effectively segregated and concentrated using low cost gravity methods, such as jigs, sluices, shaking tables etc.

Microcrystalline gold is common within primary deposits. Grains of gold are disseminated and intergrown within a quartz gangue or locked within sulphide minerals. Coarse grains can be liberated by crushing and grinding followed by concentration using gravity concentrators. If the ore consists of very fine grains extraction with sodium cyanide or amalgam is necessary.

Gold contained within sulphide minerals is present as small grains and particles within the crystal lattice of the mineral. Many primary deposits consist of disseminated grains of pyrite, chalcopyite, arsenopyrite and/or pyrrhotite containing significant amounts of gold and intergrown with gangue minerals. Sulphide minerals cannot be concentrated by gravity methods due to their low specific gravity. Froth flotation is common, followed by treatment with sodium cyanide to remove the gold. Such mining methods are expensive and can only be used on large deposits, however low grades can be worked.

Gold also occurs in compounds of gold and/or silver with tellurium. The tellurides, calaverite and sylvanite are mined for their gold content. They are quite rare, however, have been mined in Kalgoorlie.

5.0 GOLD ENVIRONMENTS

Gold occurs in alluvial, eluvial, supergene, quartz vein and stockwork, shear related and hydrothermal replacement deposits. In the general sense, alluvial refers to eluvial, colluvial, fluvial and lacustrine deposits but is restricted to the traditional meaning of stream and lake deposited gold here. Alluvial, eluvial and supergene deposits are secondary deposits formed by reworking of primary deposits. Quartz vein and stockwork, shear related and hydrothermal replacement deposits are primary deposits formed by the direct precipitation of gold from hydrothermal solutions originating in the earth's interior. Alluvial and eluvial deposits are collectively known as placer deposits. Large, continuous quartz veins are known as quartz reefs and all other large primary deposits are usually referred to as lodes. Alluvial deposits are formed by the mechanical accumulation of grains, derived from pre-existing rocks, in streams and lakes. Eluvial gold is deposited on the surface by the downward movement of material, via gravity processes, from the source which is situated above. Supergene deposits result from "in situ" weathering of mineralised bedrock which leaves behind a residue of weathered bedrock, primary and secondary ore in the weathered profile. Quartz veins are formed from hydrothermal solutions which intrude the country rock along fractures and faults. Lodes consist of a closely spaced network of quartz veins and veinlets. Shear related deposits form during shearing of the host rock along planes of stress. The associated hydrothermal solutions form gold bearing alteration haloes around the shear zones. Hydrothermal replacement deposits are formed when hot aqueous solutions react with and replace the host rock.

Alluvial Deposits

Alluvial deposits consist of hydrodynamically accumulated gold by streams and lakes. They occur on the surface, just below the surface or deeply buried. Ancient stream channels that are deeply buried are called deep leads.

Gold and heavy minerals, such as magnetite, ilmenite, zircons etc. have high specific gravities; therefore, they will be transported within the base of flowing currents where they will be trapped by irregularities in the channel base or changes in current velocity. In present day channels, the heavy mineral fraction, including gold, will accumulate in pools and in cavities, fractures, depressions, behind ridges and boulders present in runs between pools. Gold will also occur in buried channel alluvium below the present river bed. Basal channel deposits will contain the most gold. These rest upon the bedrock. Other channel base deposits can occur between the surface and bedrock where they are marked by beds of coarse sediments, pebbles and conglomerates. Gold and heavy minerals will be much finer grained than the light fraction. This is due to their density and size relationships, expressed as their hydraulic ratio. Consequently, fine gold and small gold nuggets will be found with coarse sediments, pebbles and conglomerates.

Another area of heavy mineral accumulation is the point bar. A point bar is formed on the inside of a bend in a meandering stream. Current flow is strongest on the outside of the bend, decreasing inwards. As a result, heavy minerals will drop out of suspension on the inside of the bend, or point bar, where current flow is least. As the stream migrates laterally, increasingly finer grained material is deposited until the channel is finally covered by fine grained alluvium. Stream channels that migrate laterally form widespread alluvial deposits that may contain gold in the abandoned channel base or point bar.

Eluvial Deposits

Eluvial gold is deposited by gravity processes on the surfaces of hills, rises and flat lying areas. Rainfall assists by carrying the surface material, or float, downslope. Eluvial deposits consist of the unconsolidated rock fragments and soil lying on the surface. It is derived from quartz reefs and other mineralised deposits (supergene, quartz reef and lode) located above. Deposits of transported material containing gold also form on the surface of hillsides where it is concentrated at changes in gradient, such as, the base of a hill. Technically, this hill wash is referred to as a colluvial deposit but is included with eluvial deposits here.

Supergene Deposits

Supergene deposits include both secondary and primary gold that occur in the weathering profile from "in situ" weathering of an orebody. It consists of chemically altered primary grains and nuggets, secondary grains and unaltered primary gold which may overly auriferous bedrock. Supergene gold, as it is popularly known, is the chemically precipitated gold grains and nuggets deposited within surface ironstone's, including laterite, of the weathering profile. Aqueous solutions travelling through the weathering profile transport and concentrate the gold element at or above the water table. Chemically reworked and physically transported primary grains and nuggets are present in the surface and near surface laterite and soil. Secondary gold, formed by chemical precipitation, is dispersed within the surface laterite and deeper saprolite of the weathering profile. Below the water table, unaltered primary gold, within the orebody may be present. Rich deposits, such as the "Rabbit Warren" gold find, near Leonora, have been found by the metal detecting prospector in WA.

Quartz Reefs and Stockworks

Auriferous quartz veins and stockworks containing free gold are keenly sought after by prospectors. Quartz veins originate from hydrothermal solutions being injected along fractures and faults in the country rock. The source of these hydrothermal solutions varies. They may be sourced from rising magmas that crystallise to form igneous rocks. The solutions left over are injected into fractures and faults overlying the igneous bodies. They may also originate from a deeper magma source or metamorphism of the surrounding country rock.

Fractures and faults cut the country rock at various angles and in various patterns. Consequently, the infilling quartz veins cut the country rock according to the pattern of fractures. A concentrated network of gold bearing quartz veins forms quartz stockwork deposits. Widely spaced networks of quartz veins are known as vein sets. Saddle reefs form when quartz veins are concentrated in the apex of an anticline.

Quartz veins are classified as hypothermal (high temperature), mesothermal (medium temperature) or epithermal (low temperature) veins. Hypothermal veins are deposited at great depths (>3600m). Epithermal veins are deposited near the surface ( Gold is not only present within the quartz vein itself but also in the altered zone of wall rock associated with quartz veins. Gold occurs as free grains in quartz veins and submicroscopic particles within sulphide minerals. The auriferous sulphide minerals are concentrated in the altered zone of wall rock adjacent to quartz veins and within the quartz veins themselves.

In the Yilgarn Block, most auriferous quartz veins are contained within mafic rock types (particularly meta-basalts, meta-dolerites, amphibolites) within volcanic dominated greenstone belts. Ultramafics and felsic volcanics also contain gold deposits (in fact, all rock types are represented). Auriferous quartz veins are mainly controlled by shear zones and faults, particularly where faults cut competent (brittle) beds, such as dolerite, contained within less competent country rock. Vein type mineralisation occurs at Kalgoorlie, Leonora, Wiluna, Cue, Mt. Magnet, Sandstone, Marble Bar etc..

Other

Shear related, Banded Iron Formation hosted and hydrothermal replacement deposits also occur (listed in decreasing abundance). Shear related gold mineralisation consists of alteration haloes (a form of replacement) around zones of intense deformation (shear zones), formed from the reaction of hydrothermal solutions with the wall rock. Gold is present as submicroscopic particles within sulphide minerals that occupy the alteration haloes. Quartz veining can also be present.

B.I.F. (Banded Iron Formation) hosted deposits are an example of host rock control, being restricted to a B.I.F. unit. They contain either replacement style or auriferous quartz vein mineralisation. In replacement style B.I.F. deposits, hydrothermal solutions transport the gold element along faults, forming auriferous deposits by replacing magnetite and carbonates within B.I.F.. At Hill 50, near Mt. Magnet, gold is concentrated along northeasterly trending faults cutting the Banded Iron Formation. Gold is present as submicroscopic particles within sulphide minerals plus/minus free grains. The sulphide minerals replace carbonates and magnetite within B.I.F.. Auriferous quartz veins, within B.I.F., occur in the same fashion as those described under Quartz Reefs and Stockworks. These deposits are entirely restricted to a host B.I.F. unit.

With hydrothermal replacement deposits, hydrothermal solutions react with and replace the host rock, forming massive or disseminated gold deposits. In the massive style these typically preferentially replace a specific bed. This style is called stratabound as it is restricted to a single bed, or stratum. These can occur in combination with the deposit styles described above.

6.0 PROSPECTING METHODS

In the early days, prospectors adapted their equipment to environmental conditions so that dryblowers were used in dry areas and hydraulic concentrators in wet areas. Today, metal detectors have superseded the dryblower as the major prospecting tool. The gold pan and sample mill also have their uses.

Metal Detecting

The abundance of iron oxides on the surface of W.A.'s goldfields caused many problems for the first metal detectors. This led to the introduction of ground cancelling machines in 1975. They proved effective and became popular, although there are still areas where ground cancelling machines cannot operate.

The metal detecting prospector is concerned with alluvial, eluvial, and supergene gold. In the Yilgarn and Pilbara Blocks, these occur in linear greenstone belts. Areas that have been dryblown by the early prospectors mark surface gold producing districts. Many nuggets have been found on and adjacent to these dryblowing patches. Together with the geology, they should be regarded as initial guides to metal detecting areas.

Alluvial gold can be found in the small seasonal streams that cut these areas. Basal channel deposits concentrate heavy minerals and are the most prospective deposits. Laterally migrating streams that change course regularly will contain gold in the abandoned channel base and point bar. These deposits will occur in the present day stream channel and immediately adjacent ground.

Eluvial gold can be found on low hills, rises and flat lying areas adjacent to the above locations. These are often covered with quartz and ironstone rubble. Eluvial deposits are concentrated at a change in gradient, such as the base of a hill.

Supergene deposits are found on low hills or flat lying areas that have developed laterite profiles over bedrock. The occurrence of supergene gold is difficult to predict since it is controlled by a complex combination of processes. It is generally present above weathered orebodies where it is concentrated and deposited by certain solutions travelling through the weathered zone. Secondary gold occurs in the surface laterite and deeper saprolite of the weathered zone (laterite profile) and consists of dispersed crystalline grains. Chemically altered and physically transported primary grains and nuggets, derived from the original orebody, occur in the surface and near surface with the secondary deposits. These are the main targets for metal detector operators. Weathered bedrock is also often covered by thick sequences of transported overburden (sand sheets, alluvium and colluvium). This material should be avoided as it has been diluted and mixed. The prospector should also beware of laterite profiles developed over alluvium and colluvium instead of bedrock.

In most situations, alluvial, eluvial and supergene deposits will only form over bedrock or residual laterite profiles. Exceptions to this occur when alluvial and eluvial systems are fed from these areas or where deeply buried ancient river channels exist.

The beginner should locate ground that is not heavily contaminated by iron oxide or ironstone nodules that play havoc with the detectors audio. Even so, the ground cancel will have to be adjusted as the prospector moves over new ground. Audio drift or badly erratic audio signifies that the ground cancel needs adjusting. If the ground cannot be compensated for the prospector should move to a new area.

"Hot rocks" are always encountered by the prospector. These are concentrated forms of magnetic or conductive iron oxide that behave in a similar fashion to gold. Mostly, they will give broad signals. To test whether a "hot rock" contains appreciable amounts of gold, switch to the ferrous target identification mode of your detector. With other detector types that do not have a ferrous target identification mode, the hot rock can be cracked open and both halves tested. If both halves give the same response, it can be discarded. Of course, it may not contain any gold, it may just be a lump of iron oxide.

Gridding is employed to comprehensively cover a section of ground. After a nugget has been found, the area should be gridded and explored thoroughly. This is done by marking a rectangular grid with a pick or trailing a chain. A grid is formed by marking the corners of a 10m by 5m rectangle. Next, the ends of the rectangle are marked off in one step (1m) intervals. Detecting is started at one corner and continues along the length of the rectangle. When this is completed, the operator moves to the next grid mark and follows this lengthwise so that he eventually moves across the whole of the rectangle in 1m intervals. Even when an area is gridded it is possible to miss gold. The best solution is to slow down and detect carefully.

Dryblowing and Hydraulic Concentrating

Dryblowers and hydraulic concentrators are used to recover fine gold and nuggets. Consequently, alluvial, eluvial and supergene deposits, which are most likely to concentrate fine gold, are the main targets.

Alluvial deposits are restricted to present day stream channels and immediately adjacent ground. The latter is deposited by migrating stream channels that change course regularly (being deposited in the abandoned channels). Basal channel deposits usually contain most of the gold. These are marked by conglomeratic or coarse grained beds in the subsurface or along deeply cut banks. Places to look for alluvial gold include creeks and gullies along hill sides and in depressions between hills. Eluvial deposits occur on hillsides and in depressions between hills.

Loaming

Loaming is the technique of systematically sampling and testing soil for particles of gold. Loaming is carried out to locate and test gold deposits and trace shows back to their source. Loaming using gold pans was widely employed by the early prospectors. Today, sampling machines can be used instead of gold pans to test soil samples for gold. Automatic gold pans (concentrating wheels) and small, portable dryblowers are two examples.

Prospecting for Quartz Reefs and Other Deposits

Reef prospecting involves locating gold bearing quartz veins. Most of the accessible reefs have probably been found by early prospectors and explorationists; consequently, remote and poorly outcropping reefs are more likely to be found. Today, in the short term, this form of prospecting is not as rewarding as metal detecting.

Surface weathering of outcropping quartz reefs distributes gold away and downslope from the reef, resulting in the formation of alluvial and eluvial deposits. Consequently, it is possible to trace the alluvial or eluvial deposit upstream and upslope until the source reef is located. Often, the reef has been completely weathered away, leaving only alluvial and eluvial deposits.

Once a quartz reef is located, it may be rewarding to follow the reef along its length searching for auriferous locations. Gold concentrations can increase and decrease along the length of a quartz reef.

In areas that are poorly exposed, reef prospecting is mainly restricted to the low hills and rises, where outcrop is best. In deeply weathered areas, the surface expression of quartz reefs will be in the form of supergene deposits (described previously). The presence of gossan is an indicator to an underlying orebody. Gossan is the weathered product of an orebody and is stained various colours from the oxidation of ore minerals. It generally consists of iron oxide minerals with a relict box work texture left behind after the removal of cubic pyrite. Since pyrite is often associated with gold deposits, gossan may indicate the presence of an orebody.

Within greenstone belts, mafic rock types should be targeted as the most likely host rocks. Meta-basalts and meta-dolerites are common host rocks; however, virtually all rock types are represented. Auriferous quartz veins are mainly controlled by faults and shear zones. The major regional faults and shears are barren of gold mineralisation. Secondary (and later) faults and shears, leading off the regional structures, contain major quartz reef and lode deposits. Alteration haloes around quartz veins and structures (faults, shears and fractures) are indicators to gold mineralisation (particularly the presence of iron sulphide minerals). Gold is present as submicroscopic particles in sulphide minerals (pyrite, pyrrhotite, chalcopyrite, arsenopyrite) plus/minus free grains in veins.

The best method for correctly identifying sulphide minerals, particularly microcrystalline grains, is polarized light microscopy (petrography). A petrography laboratory routinely does this type of work for a moderate price.

Whenever quartz veins or zones of alteration are encountered in the appropriate geological environment they should be sampled. In some cases, fresh bedrock will not be preserved in outcrop. Laterites, the weathered product of fresh rock, are most common. In some situations, it is sufficient to sample laterite, provided the laterite profile is residual (overlying bedrock) and unmodified, since gold is fairly chemically immobile and resistant to chemical weathering, some residual gold will usually be preserved. This will vary from area to area according to the degree and type of weathering. One disadvantage is that the original rock texture is obscured by weathering; therefore the prospector cannot be certain of the rock type being sampled. Once the sample is obtained, a sample mill or dolly pot is required to crush the sample. The sample can then be panned to determine weather any free gold is present; or preferably, samples can be assayed by a lab (this would not be of interest to the small scale prospector). If the sample gives a significant result, it can then be examined microscopically to determine the nature of the ore (whether as free gold grains or in specific sulphide minerals).

Geochemical Prospecting

Prospectors with some vision and adequate resources prefer geochemical testing of soils and rocks to the loaming technique. Geochemical sampling can identify and locate deposits with poor surface signatures, such as, when gold particles are present in insufficient quantities or coarseness to show up in a gold pan or concentrator. Geochemical prospecting is carried out to locate hidden orebodies that are without visible surface indications or to define the location, distribution and size of a known deposit. With this type of prospecting, samples are collected and sent to a lab where they are analyzed for gold and elements associated with gold (pathfinders, particularly As). This type of prospecting can identify a variety of deposits- quartz reef and lode, supergene, hydrothermal replacement etc.. Soil sampling is done to locate and analyze the distribution of alluvial and eluvial deposits or locate anomalies that overlie hidden orebodies. Geochemical sampling of outcrops can be done to determine their gold content. Soil sampling or stream sediment sampling can be carried out to analyze gold or pathfinder elements. For detailed evaluation of prospects, contour maps can be drawn to show the distribution of elements. These may show the distribution of alluvial and eluvial deposits or the location of anomalies, indicating the presence of an orebody (where elements are most concentrated): for example, a reef.

Geochemical sampling permits accurate estimation of the grades and reserves of a gold deposit.

For some deposits containing microscopic gold (some shear related, hydrothermal replacement, quartz reef and stockwork deposits) geochemical analysis is the only method able to identify them.